You can fit an 8 valve 4BT quite easily. The 6 cyl is extremely tight and requires a lot of compromises/work.

Newer axles
http://www.stovebolt.com/techtips/bigbolts/p30swap/
Another possibly better idea for front disc brakes on original axle.
https://www.stovebolt.com/ubbthreads/ubbthreads.php?ubb=showflat&Number=725572#Post725572

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-