You will not find an unmolested, good running original engine. My suggestion would be a ZZ4 crate motor from GM. If you prefer, they also sell long blocks. These crate engines run from 250+ HP (the ZZ4 is 350 HP) up to 430 HP.

You can swap valve covers and other vintage acceesories to make it look like '50's or early '60's engine and still have the benefit of a 2/24 warranty.