I was thinking a 383ls motor but I guess a 421 would be fun. I like NA, but either of your choices are good ones.

I agree with show and go, drop it first, do your roll pan, and whatever else you don't need much down time for. don't tear into the swap untill you have EVERYTHING for it. motor, headers, wiring harness, intake, mounts and everything else.

this will take you a while to piece togather, so I would make it look good in the mean time