I believe the cab is a direct bolt on, as they are identical between GMC and Chevy.
I suspect the radiator support will bolt to the frame the same because the fenders bolt to it, and the fenders although different by a light hole will interchange. That leaves the bed, and I bet it is the same or close enough to make it work with out much trouble.

I recommend you get the new frame, and not cut the other one.

Grigg

•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-