One is a 1966 model year head ( 2536178 ) and the other ( 2658920 ) is a 1967 model year head; they both cc out the same, 57 to 64.5, valve size is the same at 1.75 intake, 1.5 exhaust, there is no real difference. Reference "How to Hot Rod Small Block Mopar Engines" by Larry Shepard
