In my testing X heads flow the best ootb.
They have a better shape to the intake port SSR, and as a result I also find them to be the easiest to get the flow numbers into the 250-260-270+ range.

On a totally stock 340 package, I can see where there would be essentially no performance difference.

The main difference between them is, the X head intake port was designed around using only a 2.02” valve.

The J head intake port was revised(primarily in the bowl and SSR area) so it could facilitate the use of either a 2.02” valve....... or a 1.88” valve.