[argonaut]

The reason the older truck bodies don't fit correctly on the newer frames is due to the change in design of later model frames and cabs in order to fit the saddle tanks. In the later models (73+) the cab floor under the seat is much higher to clear the front portion of the tank. Similarly the drop section in the middle of the frame is elevated in this area to support the fuel tanks and as also mount the raised area of the floor more efficiently.

Our old truck cabs aren't made to fit this way. So the cab must sit higher.

I will say though that the steel used to form the frame channels in late model trucks is better quality and often thicker (depending if it's a 1/2, 3/4 or 1 ton). While this may not matter to most 1/2 ton guys it might matter to those building a 4x4.

I put later model suspension and drivetrain on my 66 k10 frame. If I had to do it over again I would have just built a new frame from scratch using my original frame as a reference.