The intake port of an X head is slightly different from a j or any other prod. LA head. There is a lot less of a hump coming off the wall and the turn has a longer approach ramp. I wanna say I really see no difference in the exh port between the two j & x. Imo the 974 has thee best exhaust port because they Incorporated the air injection bosses and right next to the ssr unlike later versions that are a good 5/16 or more away from the exh ssr. 308 IS a great exh floor shape...but imo the port is too small. If you want to do LESS WORK and run into LESS PROBLEMS with TOO MUCH AIR SPEED...you would port an x head over a 308 head.