what are x heads and how can you figure out if you gottem?

those are arguably the best factory 340 head there was. They all came with 2.02 intake valves. 1.60 exhaust. They were only on the high compression 340 engines, and nothing else. In 71' the went to the "j" casting, which can also be found with the big 2.02 valves, and also the smaller 1.88 intakes valve as well. The "J" castings were prone to cracking between the valves when overheated. I'v seen a lot of them do this over the years, in our machine shop.