Xs are fine keep em. everyone on the web is a racer so keep that in mind. They will say xs are junk to buy aftermarket aluminum. If I were racing id do that. the most I have in race parts is a single plane intake and a 750 holley, maybe my 3 91 rear could be considered race but it came that way from the factory. The xs in my opinion work with the positive deck height pistons for that original early 340 compression ratio. Js are late 340-360 heads and some TA heads were J castings. Some say Js came from the factory with 2.02s but a book i have says they were all 1.88s. X heads were all 2.02s. X heads are collector pieces for a correct resto they are great. Js are also great and it is said they flow much better with 2.02s.