Cool idea. This is the exact set up I plan on doing in my third gen firebird. From what I understand is you can bore a 5.3 iron block to accept LS1 pistons and rods. that's I think 3.90xxx, or in that range. Make sure the 5.3 is iron though, because the aluminum 5.3s have a very thin wall and unless you plan on resleeving, you can't bore them out at all. Also, the iron block is a little bit stronger so if you plan on going forced induction at any point, it would be a good place to start. People have been boring out their 5.3s to accept forged LSX pistons and rods for a total engine build of under 3 grand. Also, with the 5.3s smaller ports you get a higher compression ration ( people often swap 5.3 heads onto LS1s/2s for higher compression). With this setup, expect ~400-415 RWHP with around equal TQ.