Usually 4 speed cars. I have seen this also. X heads were the best untouched heads, back when you could not port heads in Stock and Super Stock Class Racing. J's / U's were second Best. All this is according to Chrysler Racing. From my experience, there is not much difference between the two, although I prefer X heads. J heads came out on the "New 360" in 1971 with 1.88 intakes in 2 barrel form. Exit X heads. Used to buy J heads cheap off B and C bodies. Clean them up, mill them .020 and .019, install 340 springs, grind the valves and seats and you are good to go. 360's did not get any High Performance parts till 1974. The J / U castings are all the same, only the post machining is different.
