They were so different, that even chrysler didn't give a crap in 1970 when 340 cars were rolling off the assembly line with a J head on one side and an X on the other.
I was looking at some of the flow numbers posted a few years back by Shady Dell and also one of the rags - maybe Mopar Muscle - I'd have to dig it out but I can see the point of view that there's not enough difference to make a difference.
