Are you looking for quick spooling or max power? For quick spooling I'd go with a 70mm, max power 80mm or up, good compromise would probably be a 76mm.

FYI, 6.0 iron block and 243 heads is going to have a pretty high compression ratio with stock pistons, which will limit your boost potential. I'd think about swapping back to a set of 317 heads if I were you.