X, J, or 308?

Ive flow tested some of the heads u mention.

''J heads'' in general can cover many casting numbers, the 1st version came as casting #3418915 and went through to #587 and 974# then to #596. The main diff between the 1st gen and later following castings are the exhaust ports where the factory was getting ready for air injection/smog requirements handed down by the epa and so they added bosses for the air injection ports to be drilled...though many were not=#974/587 both could have come with or without the air injection ports drilled. That boss can help bring up the flow number in the lower lift if properly blended.fyi

The 1.88 int valve 915 and 974 versions that I have flowed go around 210/215cfm and 140/145cfm exh @.500 lift.

I have flowed 'as cast/machined' 2.02 version #915's and they went 228/[email protected] & up . The '#587'...now the diff is who did the valve job here, cause ootb from aerohead machine they only went 203cfm....due to a crummy throat cut 'mainly', there was way too much under the seat for them to even go stock numbers...imo

fwiw.

I would go with either, and if want to go wild on them....have seats installed on both int and exh to raise the valves up and lengthen that short turn and then port them.