Cylinder head choice

Oh, and since you asked about the difference in X and J heads; X heads were the original 340 head- bigger ports, bigger valves among other things. Js were basically the same, used on later 340s and early 360s; but some used 1.88 intakes instead of the 2.02s of the X heads. Flow and performance differences between the two similarly-equipped heads were pretty negligible in stock form. The Xs are more coveted and pricier.