looking to find specs on 340-X heads.

Without using t/a heads or tubing and offsetting the pushrods about the max you are going to get out of an x head is 284 cfm, and at that level there is a slight difference. About 12 cfm. It was explain ti me as the x head was cast with the big valve in mind so the water is farther back unlike the j ect. But remember that is at a level no one should really go. Unles your class forces you to.