Difference in X heads & J heads

The biggest difference I have seen between X and J heads, is about $300 a pair! LOL. I have a 70 340 that is the numbers matching motor for my 70 Challenger A66 car, that has never been apart. It has a J head on 1 side, and a U head on the other.....both 915 castings with 2.02 intake valves.