The 'J ' castings are noted for cracking between the valves, if they are overheated. The "X" casting was a better peice. and is considered the best factory production head, and always has been.
Other then the larger valve, what made the X a better casting then the J, are you talking "flow" or just structure?
I've had both & IMO there was really no difference, they both performed well, My X heads ran better then the Js in the upper RPMs untill i installed 2.02s in a set of Js & never noticed a difference in power. One of my fastest 340s had those 2.02 Js with stock ports & it was plenty fast, was a stock bottom 72 block & a small cam.
Nothings beats the 360 magnum heads as fare as cracking between the seats, I myself never came across any crack'd Js & i've had a few. Not saying it doesn't happen.