No the factory J heads with the big valves are the same thing. They came on the later "early" 340s. IF that makes sense. I THINK the J heads began in 1970, but I wouldn't swear to it. So if you have "any" of the open chamber heads with the 2.02 and 1.60 valves, the X heads don't really "win" anything other than being original to the early 340s. But still, I agree 100% that in the right situation, the X and J heads if original big valves heads can bring some dough to a guy looking for restoration purposes. But take "that" out of the equation and they are all about equal "except" for the valve sizes, which can be upgraded anytime. I'll put it this way. If I had a 68 340 car and was doing a nut and bolt resto, I'd sure pay a premium for X heads. But only because I needed original number correct castings. It's far from the fact they are anything "special", because other than being correct for early 340s, that's about it. They won't add anything extra to any small block that has any other "letter" head with the big valves.
