I have used this place for corvair parts at the shop i work at and they have some very interesting things for the corvair transaxles. check out this link to see their saginaw 4spd tocorvair diff kit. they also have differential upgrades also. Not sure if this can help us ropeshaft folks but i thought i would throw it out there.
http://www.corvair.com/user-cgi/cata...O&page=OTTO-35