If you've got a 55 2nd. series, and it had a V-8 originally, either the 305 or the 350 should bolt in on the original mounts. The big block will require a little more effort, or if you want to go to a newer transmission/rear end assembly, you'll need to fabricate some mounts.
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!