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

I would say nothing but an X or a J casting. No difference. Edel makes 2 heads for a 340 because there were 2 piston deck heights used and the first edel head was a closed chamber design. 340 branded head was open chamber for pop up early 340 piston. 2.02 valve in anything lower than 6500 rpm is a torque/HP killer. 2.02 was only offered to keep up with the Jonses (Chevy) valve size offering. 1.88 in a street car.
..."On the 340 engine you should use the 1.88 intake valve to keep the port velocity up in the larger runner. The same modifications that were done to the 318 heads will apply here also. A good 340 head with the 1.88 valve should flow in the 235/240 range @ .500 lift and 160+ @ .200 lift if the port is properly done. The same air flow will apply to the 360 engine. The specs stated above would be a very good street/strip head and would satisfy most 340/360 engines.

There is no need for a 2.02 valve until you're racing at high RPM's above 6500. The valve size is just too large for any low RPM engine, and bottom end TQ and throttle response will be suffer...."

http://www.bjrracing.com/small_block_heads.htm