How do heads that flow 270 hit a wall at .510? Who flowed them? That set of numbers is not accurate. Reversion is not a head issue, it's a cam issue. But they do run into choke when things get too fast thru the port or turbulence is too great. In any event, a head that flows 270 is a very strong flowing iron head. Hell, it's pretty good for any aluminum port. If the head loses over .500, your max flow number is at least 30-40cfm too high. As far as stroking... did you sonic test the bores? the vast majority of 340s cannot be bored over .040 and have enough meat where they need it for a strong 4" engine. Plan on clearancing the block. Youmay not need to. But I'd plan for it rather than hope it fits.