Your alternator diodes are fried. Same thing happened to me. When the diode fails, it allows reverse flow from the battery to the alternator with the engine off. I'm sure mine died because with u/d pulleys, it never maintained a full battery charge. Alternators aren't designed to charge a depleted battery, they are to maintain a fully charged battery while everyday driving with the lights, heater, air, etc. operating.
Being an old and lazy s.o.b., I installed a powermaster 140amp one wire alternator in my '93. Fits the mount with no modifications, one wire to the battery (supplied with the kit), tape up the two plugs and tie them back out of the way. It took longer to make it look decent than to install the whole thing. (and the stock volt gauge still works, as it's not tied into the alternator circuit). Lots of choices out there, powermaster, 3g, paperformance, etc. Stock sucks on pulleys.