X heads on a 360?

Here is another thought..........sell the "X" heads to some one who desperately has to have them....use that money to rework your "smog" heads; which by the way are really NOT THAT BAD OF A HEAD; the biggest difference between the two of them is intake valve size, something easily corrected.........done right, you could end up with money left over.........food for thought

To the op if your not looking for max porting/hp ALL 360/340 ARE THE SAME basic casting. some have 1.88 some are cut more to except a 2.02 intake .
If your smog head have the air pump ports drilled out. You may need to drill and tap the airport in the exh port and put a plug in it.

On the ported 340 head. they may have been decked lowering the cc in the combustion chamber(this is a plus) but my cause" intake to head" fitting, an issue.

Now that you have this info.
I would bolt then on the X heads and save your "smog heads", as a spare just, incase the ported X head crack.
There should be a noticeable difference in performance.

If they were ported badly, and run badly on your car???? Pull them, sell them, and have a good 3 or 4 angle valve job done on yours and you will have a Hp improved Smog Head.:)