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You are here: Home Tech Articles & Tutorials  Body & Paint Chassis Comparison - '67-'72 vs. '73-'79
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Chassis Comparison  -  '67-'72 vs. '73-'79

 

     One of the more frequently-asked questions from owners of truck owners involves a body swap of their '67-'72 body onto a '73-'79 chassis. This article will explore the similarities and differences between the two.

     NOTE: All references to 'bumpside' and 'dentside' trucks in this article refer to the differences in the body-side contours. The '67-'72 bumpside trucks have a convex contour (a 'bump') running down the body's beltline, whereas the 'dentside' trucks have a concave contour (a 'dent'), as pictured in Fig. 1 at right.


Fig. 1 - The '67-'72 truck on the left is a 'bumpside' and the '73-'79 truck on the right is a 'dentside', both for obvious reasons.

     Swapping the bumpside body onto a dentside chassis is often a inexpensive and less labor-intensive method (comparatively speaking) of upgrading an older truck to power steering and power disc brakes. There are also several other advantages, such as:

  • Allowing you to have a rear-mounted fuel tank (depending on application)

  • Getting a wider differential housing for slightly-enhanced stability

  • For F100/F150 trucks with a 9" Ford rearend, the pumpkins usually had a stronger 31-spline setup, as opposed to the 28-spline used in earlier trucks.

Let's explore these in more detail:

THE REDESIGNED CHASSIS

     When Ford introduced the Twin I-beam front suspension in 1965, the front track width increased by several inches over the previous Mono-Beam solid axle setup. However, they continued to use the same-width rear differential, resulting in a mismatch between the front and rear track widths of the bumpside-era trucks. However, this was corrected beginning with the 1973 2WD and some 4WD models. While the frame remained basically the same from the rear of the cab forward, everything from the back of the cab rearward was widened out to 38", an increase of 4". The following chart shows the widths of the rear framerails of various models:

YEAR APPLICATION REAR FRAMERAIL
SPACING
'67-'72 2WD/4WD narrow (34")1
'73-'79 2WD wide (38")
'73-'79 F100 4WD wide (38")
'73-'77 F250 4WD narrow(34")
'77-'79 F250 4WD wide (38")
'73-'79 F350 2WD Super Camper Special narrow(34")
1: I'm not entirely sure how far back that goes, as 34" wide was an SAE standard for all trucks back then.

In their 1973 promotional literature, Ford stated the wider frame spacing, in conjunction with the new long 2" wide leaf springs, improved road stability. With the redesigned frame came a new 4"-wider rear differential housing, which finally enabled the rear wheels to track directly behind the front wheels. Obviously, the leaf-spring mounting pads on the newer differential were also spaced 4" farther apart on the axle tubes.


FIG. 2 - SOURCE: 1973 Ford promotional brochure

The wider framerail spacing also allowed for a rear-mounted fuel tank in most models that "is mounted within the protection of the husky frame siderails under the rear of the pickup bed just above the spare tire". This replaced the in-cab tank, which in turn gave the driver more room inside the cab and some storage space behind the seat.

The frame redesign didn't stop with simply widening out the rear half...it was also stretched back 2", increasing the standard LWB wheelbase from 131" to 133", and the SWB wheelbase from 115" to 117", which in turn made the pickup bed 2" longer as well. (Ford had described the bumpside SWB box as a 6-foot box, but now referred to the dentside version as a 6-foot box, while continuing to refer to the LWB version an 8-foot box.)

DOING A BODY SWAP

There are many reasons given by truck owners for wanting to swap their bumpside truck onto a dentside chassis. Most are looking it as an upgrade...as a quicker way of giving their truck power steering, power front-disc brakes and a rear-mounted fuel tank. If the dentside donor is a Camper Special, then the truck is also already equipped with a standard front anti-sway bars and possibly the optional rear bar. (Bumpside trucks could only get a front anti-swaybar on the front of F350 and some F250 models, and it was a strange piece that extended between the front radius arms instead of being mounted up front of the axles, as in a car application. See Figs. 3 and 4).

Whatever the reasons, if you've decided to explore the possibility, here are some of the things you need to be aware of.

Mounting the cab and front sheetmetal is a bolt-on affair...there were no changes done to the front part of the frame to affect that. However, keep in mind that the rear cab mounts on all 2WD and F100 4WD trucks are part of the frame crossmember, whereas F250/F350 4WD trucks have a bracket on the outside of the frame. To mount a 2WD cab onto a F250/F350 4WD chassis you'll need to drill new holes in the cab in the areas shown in Fig. 7 (picture coming soon).

However, because the rear frame section is longer and wider, mounting the box will require a little work, though it's still definitely do-able.

The box is bolted directly to the frame. Because the dentside frame is wider, you'll have to drill new mounting holes in the box. In of itself this is pretty straight-forward, except that you need to consider that the factory reinforced the bed's mounting holes with hollow tubing, preventing the bed floor from distorting when the hold-down bolts were tightened up and to prevent the bolt heads from pulling through the floor. Fig. 8 at right shows these tubes on my '67 after removing the bed floor. Because these are inside the bed support, it'll be next to impossible to add these, but you might be able to reach in through the end of the bed support with something long to insert a similar piece of tubing. Otherwise, be aware that over-tightening the hold-down bolts will distort the floor and could possibly cause the bolt head to completely pull through the floor.

The other thing to consider is the difference in wheelbase. Remember that the dentside truck frames are 2" longer behind the cab. Therefore, positioning the box so that the rear wheels are centered inside the wheelwells will create a slightly larger gap between the cab and the front of the box. On the other hand, if you shift the box forward to restore the proper cab-to-box gap, then the rear wheels will no longer be centered in the wheelwells.


An example of a '72 F-250 body mounted on a '76 chassis. In this case, the owner centered the rear tire in the wheelwell when mounting the bed, resulting in a 2" greater gap between the cab and the bed.

There are two ways of dealing with the situation:

  • Simply split the 2" difference, resulting in a cab-to-box gap that is only 1" wider than factory and rear wheels that are about 1" farther back towards the rear of the wheelwell. Most truck owners who do the body swap use this method and report the difference is virtually unnoticeable on a 2WD truck and not even an issue on a 4WD.

  • The second method is considerably more work, but the end result is better. Mount the box in the full-forward position, maintaining the proper cab-to-box gap. Then relocate the rear spring mounts 2" forward, which will in turn bring the entire rearend housing with it, centering the wheel in the wheelwell. Of course, this method will probably require the need to shorten the driveshaft to compensate, and possibly the rear brake lines.


Fig. 3 - Bumpside front anti-sway bar. This style of swaybar interferes with header installation.


Fig. 4 - Bumpside front anti-sway bar


Fig. 5 - A comparison shot of the end of radius arms. The dentside version is slightly longer and has a notch to accept the front swaybar link bracket (not shown).


Fig. 6 - I couldn't find a picture of the swaybar link bracket as mounted on a truck, so here's one from a van. The bracket is identical though. You can see the notch in the end of the radius arm to accept the link rod locating pin.

[PICTURE COMING SOON]
Fig. 7
- Here are the areas where you'll need to drill new cab mounting holes to mount a 2WD cab onto an F250/F350 4WD frame.


Fig. 8 - Here's a view of the bed-floor supports, shown after removing the floor. These keep the hold-down bolts from distorting the bed floor when tightened down, as well as preventing the bolt from pulling through the floor.

 

 
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