Which Heads Do I Have?

All 340/360 heads have the same port and chamber design so the potential is the same with all of them, there is nothing special about an X or J head. The early 340 heads had 2.02" diameter intake valves verse the 1.88's in the later 340 and all 360 heads. Your heads can be cut to accept 2.02. BTW, the 2.02 is not really an advantage unless you plan to build for a lot of power (400ish).