454 and 427 are the same block, so don't worry about picking from the two. Any year should be fine. 66-69 would probably be the toughest because those were the years of the 450 hp 427. Just be sure to get yourself a good forged crankshaft. I would shy away from 70-72 because it looks like most of those had cast cranks, but some were still forged. The only way to be sure is to check the numbers. Just pull the pan before you buy anything.