X heads on a 340 HP potential???

Have a fresh set of X heads that flow 240/165 @ .500 on the bench. Been gasket matched w bowls blended a little. What kind of HP potential may these heads have on a 10.3:1 340 w/ a, MP .509, RPM air gap, and a 750 holley. Machine shop told me around 325, but i was hoping for more than that. I thought they were under rated and more like 325 stock. When these heads were bone stock they flowed 220/140 @ .500