First choice would be a diesel, a Cummins 4BT is worth looking onto.
My personal choice would be a Detroit 3-53T, that's what goes in my 52 1 ton.

Of the two choices you presented It'd stick with the 6, either the 235 or preferably a 261.
Went to a good truck show yeasterday, lots of V8 AD trucks and not much at all interesting there, but several interesting 6 cylinders to look at, most with "speed parts".

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-