Re: D-series truck
Sounds like the cooling system is restricted, probably with 36 years of corrosion. Putting a bypass from the rear of the engine will probably slow the coolant flow through the head, causing a hot spot somewhere else, if not stop flow all together (I wouldn't advise it)
Have you tried getting the cooling system power flushed?
Yes the manual gearbox will adapt to a V8 but you will need a bell housing and pressure plate.
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