72 Demon 340 J or X heads??

This subject has been discussed to no end, over and over...

X, J and O heads are the same casting number "915", "X" and "O" heads typically have the large 2.02" valves, the "J" heads also came with 2.02" valves but not all of them.

I have "J" heads on my #'s 340, they are 2.02".
Also the famous "TA" head, is a "J" head with an offset pushrod.

So likely your '72 Demon will have "J" heads, with the smaller 1.88" valve.
Compression ratio, cylinder heads, cams ...started there performance decline in '72.

Hope this helps.