X-heads vs J-heads: what's the difference??

X heads were used on 340s from 68 to 70. They came with 2.02 / 1.6 valves. The J head was from 70 to 71. They came with 2.02 or 1.88 intake valves and 1.6 exhaust valves. There is some factory parts bin scrounging / emptying so one may find these on later years than whats the norm. As far as OOTB performance goes, both heads IMO are very similar when equipped with 2.02 valves. Some may argue the subtle differences between the two, but the reality is they are nearly identical.