why J heads, why not x heads?????

1st The x heads have less material in the roof compared to J heads.
2nd T he exhaust floors of the X heads seem to have more short turn height, or maybe it;s just my eyes..
3rd The j heads DO have minor differences over the span of production, like the smog heads have a lil less int ssr/floor hump, the smog heads do have floor bumps/air injection bosses with some undrilled. I don't believe the bump is really worth much if any.jmo
4th The 1.88 head as a start is better to work with when porting due to the far side throat cut from the factory-they removed a lil too much on the 2.02 heads JMO I'd rather start fresh.
The diff between the later j and earlier J is in full ported form..the early J head will make a peek of around 20cfm int over the later in most cases.
as in...all the ported later 70's heads quit around 250's while J heads go 270+, at least from what I've seen