Priming the Interior Floor & Heidt’s 4-Link Rear Suspension Mount Mock-Up

With the floor installation finally complete, and my summer distractions winding down, I was able to make a bit of progress on finishing the floor as well as starting to mock up the Heidt’s 4-link rear suspension system that I am installing in my car.  Summer heat has played a significant role in slowing down progress as the temperatures have regularly been in the mid 90’s or above, making even brief forays in the shop a rather uncomfortable experience.

The first order of business I wanted to tackle was getting the interior floor and firewall primed now that all of the welding and grinding work is complete.  I started by scuffing the entire interior floor and firewall area with a course scotch brite pad and cleaned everything well with PPG DX-330 Acryli-clean and let everything dry.  After mixing up the requisite amount of PPG DP40LF primer, I shot the entire interior floor/firewall area with two coats with about 15 minutes flash time between coats.  I was immediately struck by how much better the whole works looked by simply having everything in one solid color!  Sometimes, it’s the little things that keep you motivated I suppose.

With the primer work done, I switched my focus to mocking up the Heidt’s 4-Link chassis brackets in preparation for their permanent installation in the chassis.  Here is where the wheels kinda fell off the wagon a bit.

As many of you may know, the 69-70 Mustangs had the option of two significantly different rear shock mounting configurations.  Most popular is the “non-staggered” (a.k.a. standard) shock mounting configuration.  This places both rear shocks in a single plane forward of the rear axle like all other Mustangs from 64½ through 68 and is therefore the configuration that most aftermarket suspension systems are designed to fit (including the Heidt’s 4-Link).

The second, and far more rare, rear shock configuration is the “staggered” mounting configuration which places one shock ahead of the rear axle and one behind the rear axle in an effort to reduce axle tramp/hop “back in the day”.  Usually, this staggered-shock configuration was also accompanied by a rear swaybar of some dimension.  Generally speaking, the staggered-shock configuration was only found on big-block and Boss cars in 69-70. 

To achieve this configuration, Ford incorporated a frame-to-frame reinforcement panel that was welded underneath the car across the back of the axle tunnel through which the upper rear shock mount protruded.  With that, a hole in the upper axle tunnel was often crudely cut for shock mount clearance and an upper reinforcement/shock mount “channel” box was welded in the interior of the car directly above the axle tunnel and a plug was welded over the driver side forward shock mount hole to complete the chassis modifications.  As simple as this sounds, I quickly discovered there was no way possible to fit the Heidt’s 4-link rear suspension mounts to my car without some significant changes being made.

I must confess that this problem was not entirely unexpected when I decided on this suspension system.  The only real dilemma was deciding what method I would use to overcome these issues and get everything to fit as I (and Heidt’s) intended.  After many hours and days of review and thought, I decided to remove the staggered shock components as part of the rear floor repairs I would have to make anyway.  This would allow the Heidt’s kit to be installed as intended with no modifications and allow the floor repairs to be more easily accomplished in the long run.  Hopefully, the attached photos will make all of this much more clear.  In hopes that future Mustang enthusiasts will be spared these surprises, I have contacted Heidt’s in hopes that my installation experiences will provide them with enough information to make appropriate notifications in their kit documentation to help avoid any issues with future staggered-shock Mustang customers.

Once the modifications were made and the frame mounts were test fit, I was able to see how the front trunk floor patch panel would work.  In short:  it didn’t.  In fact, I have subsequently found that NONE of the available front trunk floor patch panels are made properly and will not fit!  Imagine that………*sigh*

Specifically, the front trunk floor patch has a very sharp bend in the area just forward of the front fuel tank mounting flange.  In production, this area of the floor has a nice curve formed into it that perfectly matches the curve of the outer trunk floor panels as they transition into the axle tunnel/main floor area.  The replacement panels in the open market are COMPLETELY wrong in this area and will not even come close to fitting properly.  Unfortunately, I have become somewhat accustomed to this and went about solving the problem “my way”.

I have had the fortune of being instructed in the use of a wonderful capable but deceivingly simple sheet metal working tool called an “English Wheel”.  An English wheel is essentially two steel wheels (or anvils as they are properly termed) mounted opposite each other in a very large, ridged frame that resembles a giant c-clamp.  This tool can form the most complex of curves in sheet metal panels or flatten unwanted features by simply choosing the correct anvils for the job and exercising some patience to not rush the work.

After “un-bending” most of the offending crease on a sheet metal brake, I borrowed some time on a nearby English wheel to slowly roll out the seam using two absolutely flat anvil wheels and light rolling pressure.    Total time from start to finish?  About 12 minutes.  Another testimony to the right tool for the right job.  I have GOT to get me one of those…….

I wanted to see how the front trunk floor patch would fit now that the flaw was corrected and to hopefully start establishing the proper curve to the panel that was required for correct fit.  With a little hand forming and the help of several locking panel clamps, I found the fit to be quite promising.

Next in the work plan is to prime the underside of the floor where the subframe connectors are located along with the subframe connectors themselves.  After that, I will return to fitting the rear suspension forward mounts to the chassis and prepare for final installation of the mounts and subframe connectors.  The goal is to have this work completed by the end of August in preparation for the rear trunk floor and tail light panel repairs in early Fall.

Scotch Brite does a nice job of scuffing the e-coat on the floor pan in preparation for primer.  Here's the front floor and firewall ready for priming.

Here is the rear floor section scuffed and ready for primer.

It is really amazing how inspiring it is to see everything in a single, solid color.  Two coats of PPG DF40LF did the trick.

Here is the rear interior floor section in fresh primer.

 The Heidt's 4-Link rear suspension mock-up was full of challenges.

Most 69-70 Mustangs have non-staggered rear shocks which mount in the triangular recesses shown at the top and bottom of this photo.  To the left, you can see the staggered-shock reinforcement and mount that is unique to Boss and big-block 69-70 Mustangs.

Here is the driver side of the staggered shock reinforcement panel that is welded to the bottom of the axle tunnel and to the inboard rear subframes.  This structure prevents the Heidt's frame brackets from sliding over the subframe rails per the installation instructions.  This is a problem unique to staggered shock cars in 69-70 only.

Shown here is the factory rear shock mount on a staggered shock car.  Note how the shock mount comes through the axle tunnel from the inside of the car.  Also, note the crudely cut pass-through opening in the axle tunnel to make room for the mount.  Yuck!

Here is the passenger side weld on the staggered-shock reinforcement.

At the top of the axle tunnel is the upper reinforcement/staggered-shock mount "channel" (the vertical box structure in the middle of this picture).  To the left you can see the original upper shock mount locations normal to a non-staggered shock car (the oval holes to the left of the image).

Starting with the lower stagger-shock reinforcement, I cut each spot weld with my Blair spot weld cutter and used my pneumatic "death wheel" to cut cleanly through the weld beads at each end next to the frame rails.  With a little prying, the entire panel came off intact.  Afterward, I moved to the inside of the car and removed the upper mount and channel box .  You can see the size of the pass-through hole for the upper mount in this picture.

This is the lower reinforcement panel after removal from the bottom of the car.

Here you can see just how crudely the hole was cut for the upper shock mount.  They used a bloomin' TORCH!

With the weld areas cleaned up and smoothed, the trial fitting of the upper coilover /panhard mounts went very well.  Here is the driver side mount properly located.

And here is the passenger side coilover/panhard rod mount mocked-up.

With the upper coilover mount crossbar in place, the fit is snug, but clean on the passenger side.

Clean fit on the driver side as well.

Here you can get a feel for just how snug this system packages up against the axle tunnel and front trunk floor transition.

As you can see at the top left of the photo, the location of the original staggered-shock mount completely prevents the proper installation of the Heidt's upper coilover mount.

Front trunk floor patch test fitting

I used an English wheel similar tho the one above to roll the seam out of the front trunk patch. Normally, this type of tool wouldn't be required, but since discovering the gross error in the trunk floor patch design, I was glad to have one available.

I started by "un-bending" the crease in the panel as flat as possible using a conventional metal brake.  Then I wheeled the seam flat in the English wheel.  Looking down the panel like this and the seam is virtually invisible and the panel is nice and flat.  This is something I can work with!

In this picture, the seam can just barely be seem as a thin straight line where the original press brake die formed the crease.

The flat floor patch section was rather easily formed to the rounded contour of the original trunk floor edges using welding clamps.

Here you can see how closely the contour is matched to the original trunk floor.  I can work with this.

There will be enough metal left from the patch panel to repair the original staggered-shock mount pass-through hole as well.