There is no V8 that will bolt in, since the GMC and Chevy six bellhousing bolt pattern is completely different until the early 1960's when inline sixes, V8's and some V6's began to share the same bellhousing bolt pattern. You'll also encounter major difficulties with exhaust manifold and steering gear clearance issues. Ed definitely has the right idea- - - -a 302 or even a 270 GMC six will be almost a bolt-in swap and will provide the torque you need to keep up with traffic without butchering up the truck.
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!