I'd consider a replacement original frame, power steering, newer rear axle with fast gears and a Cummins 4BT with your choice of transmission. If Allison then the 2000 may be easier to bolt on than the 1000 that is intended for the Duramax, otherwise the 2000 is essentially the same as the 1000.

Look over here:
http://www.4btswaps.com

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-