WE just put mt semi ported over sized valves 516s on my kids 62 300.The 413 heads are super restrictive.They had 2.02 and like 1.60 exhausts.He still runs 13.6.He dropped maybe .05 changing them.He is spinning more off the line and the cam is holding it back on the top end.I figured they would be good for at least 3 tenths difference.Change one thing you have to change another.With the old heads it was done at like 5500.Now it wants to pull but the cam just wont let it.Rocky