Well since you are going to be doing a bunch of fab work to get that trans in, you might as well go Big. That smogger 440 is a good start. It's all up to your budget. Guys selling 340's think they are worth a mint...so if you go SB opt for a 360. There are plenty of BB A-bodys around. A cheap mild 440 build will make more usable power than a more expensive 408. Ask yourself what funds you want to spend and what kind of power you want to make. A simple .030 over 10:1 440 with decent heads will make 475-500hp w/o much trouble. It will take some effort to get those #'s out of a CHEAP SB.