The only difference between the Dyna III and the Dyna S is the Dyna III uses a power module between the pickups and the coils to amplify the spark, while the Dyna S is standalone and does not use a "Black Box". I can't speak for the cross-platform compatibility between the 750 and 500 engines, but if the points plates are compatible between the two, then I would expect the dyna pickups to also carry over. Let us know how it goes.