Voltage regulator upgrade? Keep frying voltage regulators with swap

Instead of the alternator side, have you checked the other side of the regulator? As mentioned earlier, it sounds like the regular employs a sense wire. Perhaps under heavy load this wire is dropping too much voltage and causing the alternator to overcharge? Just throwing ideas out there now since it sounds like you've tried most other things. About the last thing you could check would be swapping the alternator if you can find a spare just to see if it does something weird at high output requirements. Maybe it has some kind of internal fault that isn't present unless at high loads.

For what it's worth, I'm running two main charge wires. I have a 4 AWG or so wire from the alternator straight to the battery (or starter relay, same difference), but I'm also running the stock charge wire. I'm doing it this way because I still need power at the dash to run all the gauges and didn't feel like rerouting or running a new wire. The parallel charge circuits make the ammeter effectively pointless, but I have a voltmeter in a little gauge cluster under the dash as well. It seems to read on the high side of things though, so I suspect I might have some odd wiring/connections, but everything seems to work fine. Still using an old fashioned electronic regulator.