A 47-53 1/2 ton frame may be tapered from front to back. But a 1 ton and 2 ton only taper from about the drivers seat forward, behind that the rails are parallel, I have one of each.

It sounds to me like you are trying to get away from someone else's poor choice in parts and fabrication by making some awkward choices yourself.

I would recommend you find a good frame for the truck you want to play with, and go from there. With a little searching I think you can turn up a good frame for less than $200.

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-