340 Max HP Potential with X/J heads?

To the op.
If you don't have the skills or tools to port your heads. Just ask your machinist to do a good 3 angle valve job. when you get them home, remove the intake valve and smooth up the bottom cut in the throat. This is the single biggest JUMP in CFM that you will get in porting the head.
Yes there is a lot more HP potential hiding in them heads but it take a lot of time and patience to get out of them.

From that point i would jump to the iron RHS heads.
or if you must eddies that need to be sent to you machinist to have some clean up done for longevity.......
Just my .02 sence
W/2.02...When you raise the port roof enough n trim up the guide, you can almost leave the short turn barely laid back at the top after a good steep throat cut and have 260's. You have venturi it properly in such case. That's why that set of mine looked almost factory on the turn. W&C