Fuhrmann, the "Rockcrusher" was the M-22 close ratio Muncie. It was called that because the gears on it were a different angle than the M-20/21 Muncies. They made more noise than the M20/21's Alot of people think they have "Rockcrushers" but have an M-21 instead. They are actually pretty rare. If your rear gear ratio was 3.55 & higher, you had an M-20, anything lower had an M-21/22.
BTW, I would strongly recommend replacing the Saginaw, it is a weak transmission & will break easily behind a 400. What is your rear gear ratio?
