A Freightliner or Oshkosh stepvan with the 6BT Cummins may also be a good start. Check the width of the front axle compared to your fenders.
Either should come with 19.5" wheels and 22.5" are available.

Another option is swap the engine, trans, and axles you want into the old frame, there's really nothing wrong with the old ones. This is the method I've chosen and I'm pleased with it.

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-