The 261 engine, which was used in midsize and larger Chevy trucks from 54-61 will be the closest thing to a direct bolt-in you will find. The GMC 270 and 302 engines will be close, but they will require front motor mount modifications to accomodate the 2" of extra length and a relocation of the radiator forward a little if possible. The GMC engine is a direct bolt-up to the Chevy bellhousing, however.
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!