thay talking j heads the magnum head was used on both 5.9 and 5.2s with a 59ish cc chamber, probably closer to 62ish!! after a .040 cut the j's come in around 65ish depending on how far off factory was to there own specs! what lil compression is lost is not noticeable, better cylinder filling by better heads might not lose a .01 of compression at rpm!
cause it works!!! any alignment of the ports can be fixed with gasket matching! .040 off the heads ant much, and with adjustable rockers is a non issue, with hydraulic lifters it ant nuff to worry over ether! better flowing heads will always make better power!