[hatzie]

The NV3500 will handle the power output of a stock mid '80s 350. Max torque on the NV3500 is 300 ft/lb. Towing is basically out with that tranny. The gearing is more like a car tranny than a truck tranny.

The SM465 is essentially a 3 speed with an additional Low... the NV4500 has the same gearing with the addition of an OD gear.
The NV4500 is a heavy beast like the SM465. This swap can easily run into the $3000 range. That's the first reason the Gear Vendors is a good option.

You can tow with the NV4500 but... DO NOT TOW in 5th gear. This is where the Gear Vendors OD unit blows either 5 speed out of the water... You CAN tow in OD with a Gear Vendors unit without blowing the OD all to pieces.

If I had the 76 to do over I'd do a Gear Vendors OD unit...

If you must have the NV4500...
The 96+ transmission has the internal slave cylinder. The 93-95 unit has an external slave similar to the 85 but low on the RH side. Mating the slaves to the 85+ MC with a hybrid hydraulic line is part of the fun... I used a 96+ unit.
93 only has an extra low L (1st). Extremely rare so you prolly won't see one.
Make sure the *$&% salvage yard includes the bellhousing and shifter... No bellhousing? Don't be shy... Tell em why the sale just evaporated.