I changed my voltage regulator on my 68 350 because the alternator was over charging whenever the engine was running. No improvement. I changed the alternator yesterday and still no change. I'm going to pick up another voltage regulator at lunch today. Could it be that the old one was defective?
Without you taking some voltage measurements at different points, answers are only a guess. Here's my guesses:
Do you have a ground wire from battey (-) to the right inside fender?
Is the regulator grounded? If it is mounted on rubber wellnuts, it needs a ground strap to the core support.
It sounds to me like your regulator isn't grounded. This is pretty common on GMs with mechanical regulators. If you don't have the ground strap from the battery to the core support and the one from the core support to the regulator, then the car will radically overcharge and kill the battery.
When the car overcharges, it overheats the battery and warps all the plates. (the case will get all huge and round )