Why settle for a Mk3 alternator? I've been running a Mk4 alternator for a year and a half now (almost 2 years) and love it! 100 amps kicks ass! I've got a relatively big stereo system, twin electric fans (20 amp draw!), 100w foglights, and now even with all these on, and the rear defrost and wipers, the voltage level stays above 12V. It's great! The alt is an Auto Zone rebuilt unit, part number 13240 for a 1996 Twin Turbo Automatic. Cost before core is about $180.
The pulley is 6 rib instead of 5, which helps somewhat. I heard that going to a 6 rib belt instead of 7 would shorten the life of the belt, but it's been on there the whole time and it's doing fine. This will only work on a 6M or 84 or later model 5M.
If you have an 83 or earlier Supra, you'll need to remove the charge light relay and replace it with a diode. I have the info on this somewhere, and it's super easy...
The bracket mods required to do this are similar to the Mk3 requirements, and I even have the Toyota part number for the Mk4 alternator clip, which is about $10, wires and pins not included. It's 90980-11349. The guy where I got mine immediately knew which wires to get and they were about $6 apiece.
You'll also need to find a 3" or so bolt, a nut, and a bunch of washers to replace the upper mounting bolt.
I'm using a 38" 6 rib belt, from Auto Zone.
I have pics of the bracket mods and the upper bolt replacement. PM me with your email address and I'll send them along to you.