I remember going through some manuals and GMC actually recommended a hardening process called "Tuftriding" after turning the crankshafts . When the crankshaft for my 478M was turned down I requested that it be rehardened, was no problem with my machinist he sent it out for that but told me there was a newer process that superseded Tuftriding which is "Nitriding" and that's what was done on my 478's crankshaft
