Swap M21 for Saginaw

[Green Machine]

Don't hold me to this but I think the 67 and earlier cast bellhousings have the smaller diameter retainer hole, that might be an option, assuming you want to hang the tranny like original. The cast housings have no threads n the bottom tranny mount holes, so you will need longer bolts with some nuts, use only grade 8 bolts and nuts here.