Lots of Hype for nothing other than a collector of an original vehicle looking for the correct head #.
The X,J,O,U heads all flow within 5 cc of each other on a flow bench when ported and polished equally, the only advantage any of these heads have over an other is the large intake valves in the X&J heads.  

Most combos will see little gain with the larger 2.02 valves also.  

Just keep in mind, that not ALL J heads have the larger valves.  

O.K. thanks guys. One more question, I want to upgrade my original cast intake (chrysler dual plane) and I'm kinda in between two manifolds, the Edle Perf Air gap and the Perf RPM Air gap. What do you guys think.  

They are the same being that they are both RPM's just one is isolated from the hot engine, either would work well.

The plain old eddy performer 318-360 is the wrong manifold for a 340/360 unless the ports or opened up on it [they are 318 sized] it would be a miss match and really not much better than stock except it's aluminum.  

the 894 head was 340 1968-70  the 915 was 1970-72 340, 340 six-pack, and 1971-72 360 = all 915 castings started out the same-the end piece was different in how it was machined by chrysler  70-71 340 2.02 intake valve, 72 1.88  70 six-pack had 2.02 and the drilled hole for the INTAKE pushrods was moved away from the intake port so the owner could port out the pushrod hump in the intake ports, also had adjustable iron rockers   all 360 (except magnum=1.92) had 1.88 X 1.600   I believe the 915 was the intro of hardened exh seats   Also remember that the new model years came out in september, so for example a 1970 vehicle built in sept. of 1969 would have block,head,intake casting numbers like 8/21/9  

According to the direct connection performance book the #,s 3418915 started in 1970 as 340 six pack heads they when,t on to be 360 heads in 71,and 340/360,in 72.usually seeing AAWJ letters is a good start to spotting good heads