How Do I Shorten the Frame of a 1972 GMC Pickup?

Part of the Chevrolet/GMC's appeal has always been its versatility. The 1967 through 1972 generation was a particularly good example of this platform's jack-of-all-trades personality; these trucks were produced in 12 different assembly plants in North and South America in a bewildering array of configurations. To make the frame adaptable without changing its basic architecture, GM included a perfectly straight section in the middle; this feature makes changing the wheelbase a fairly straightforward proposition.

Instructions

1

Position a jack stand under each frame rail on both sides of the frame, just under the transmission cross-member and center cross-member (the one with the driveshaft hole in the center). Adjust the stands so that the pads are as close as possible to the frame and insert wooden shims to make up the difference.

2

Mark a perfectly vertical line all the way around the frame rail 3 inches rearward from the transmission cross-member. You might want to consider using a plumb bob to keep the line straight. Don't worry if the frame isn't completely level front to rear; the vertical angle will remain the same so the ends will still match up.

3

Measure and mark a line rearward on one frame rail the distance that you want to shorten the frame. Use the plumb bob here as you did on the front line. Carefully transfer your measurement to the other frame rail. Precision is critical here.

4

Use a grinder and cut-off wheel to slice right down the center of all four lines marked in Steps 2 and 3.

5

Replace the cutting wheel with an abrasive one and grind a 45-degree bevel on the edges of the cuts. Bevel from the outside on the rear cuts and the inside on the front cuts to ensure that they line up.

6

Push the two sides of the frame rail together and tack-weld the joint in four to six places.

7

Cut two 18-inch lengths of 1/4-inch steel plate, making them just wide enough so that they will fit inside the C-channels in the frame rail. Bevel the edges on one side of the steel plates as you did on the frame rail.

8

Tap the steel plate into the frame rail with the beveled edges facing towards you, ensuring that the joint in the frame rail is roughly centered behind the plate. Tack-weld the reinforcing plate in place, then move to the outside of the frame and finish welding the joint all the way around.

9

Move back to the inside of the frame rail and finish welding the reinforcing plate in place. You might want to consider running your welds in short 2-inch strips separated by 2 inches, then going back and filling in the gaps. This will help to reduce heat buildup in the metal and prevent subsequent weakening.