How much horsepower difference between the two heads (just bowl ported) on a mild normally aspirated 540? Holley 850, single plane, 9.2, Crane731,Gil offshore wet exh.

My recommendation would be to work the Merlins (assuming you have them now) to 315 thru 319 and enjoy. That should flow much better than the stock GM sq. port. Lots of guys on OSO here that can tell you lots more, and probably will. --- Jer