ive wondered about this. when my 451 cracked a cyl and hurt my ootb edelbrock head i called muscle motors about a stroker kit. i was really leaning towards a 511. they were pretty adamant that with a standard window head that 470 was the right kit, and if i went to a max wedge head then the 511 would be the better choice. i went with the 470 kit but sometimes i wonder if i should have went with the 511. over all im pretty happy with the 470, but the damn thing better run better than my old 451!