best cylinder head

So the engine is making around 380-440 HP. I've seen a lot of X heads with a "ton" of port work that flow in the 245 cfm range and stall at .470" lift.

The best head? Certainly not the best, but the simplest thing to do would be go to a ported set of RaceMaster/ProComp or Edelbrock RPM heads so you can use all the other parts you already have. They'll make a big difference over what you have now. An OOTB set of Edelbrocks will flow about what most ported X heads will. Here are the results of one set of RPM heads we did for a .550" lift cam. There are others who can get similar results.

Lift............OOTB.........Ported
.100..........61/53........68/57
.200........126/107....140/124
.300........175/142....199/181
.400........223/157....242/218
.500........248/165....273/234
.550............N/A........283/240
.600........250/169....290/245