I don't think the gear ratio difference is much of a problem. The '76 would have had a 3.91 rear--the '93 auto has a 3.73. The difference is minimal -- will give lower rpm on the highway and not enough difference to cause a problem as far as pulling power. The '93 motor is a 2.3 -- the '76 was 2.1 liter. Keep the '93 rear with it's electric speedo drive and plug the cable drive at the transmission. The main thing you'll need is a flywheel to match your ignition. Make a cut-out in the M46 bellhousing. The driveshaft from the '93 may work with the M46 length wise but the front u-joint size may not match the output flange of the M46.
I have pedal/bracket sets if you need those. I also have a good AW70 transmission in a 1989 parts car if you just want to keep it original. I also have a 561 ECU--not the best choice but it comes from a running car and you can borrow/rent it for test purposes.
If you decide not to go ahead with this project I'd be very interested in it and would appreciate a heads up. I see you are located in Westchester---I'm on Long Island--45 minutes from the Throgs Neck. Write me at--fastforwardphoto(AT)yahoo(DOT)com----or call six-three-one-968-8933---Dave