If the casting #s are different, I'd go with the Dart headpipes...if they show a different part#, .....wait a sec......Walker shows #40242 & Goerlich #93822 both '67-'74, then they changed for '75 even w/o catalyst.....but neither offered a Y-pipe for the '75 and ups...they were much longer, so no way of knowing if that was all that was changed. Hold You're old Y or pipes up to compare?