The Mercruiser 230 or 250 engines should be a drop-in fit in 63-up trucks. Just use the truck type intake and exhaust manifolds instead of the water cooled one. 62 and earlier trucks would need approximately the same motor mount and bellhousing conversion required for installing a small block V8.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!