Go back to where Hotrod mentioned the timing chain. The GM V8s are bad to jump timing teeth and absolutely will not start or run until the chain and/or gears are replaced.
There is a definite procedure for checking this out so get yourself a manual.
'31 Chevy 1 1/2 ton,
'42 GMC 2 1/2 ton 6x4,
'52 GMC 2 1/2 ton 6x6,
'53 GMC 5 ton
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