X-heads vs J-heads: what's the difference??

..early 340's were not 10.5:1,that was the factory rating that allowed you to build to that for NHRA competition.If you do the math you'll find they were around 9.6:1.
the j's or x's are excellent heads,can be bought cheap and will flow 400hp stock.
..those 74 cc X's must have had sunken valves.