Everyone tells me that x heads are better than j's, and t/a heads are better than regular j's. While I've never tried X's, Rhett has them and he is really slow. KG engines uses the same CNC program for X's and J's. I took the T/A's off and put J's CNC'd by KG and finished by PRH and picked up a good amount. It's been 10.1@132 through exhaust manifolds on a 6.5" Street tire. If I stopped running within the constraints of F.A.S.T. and put a good carb, intake, headers and a cam and converter to match I imagine it would be in the 9.5 range if not a touch quicker with stock valve sized, stock pushrod location, no epoxy or weld J heads.