Mark, thanks for the useful idea. I'll look at running a separate wire from battery minus to a frame piece nearby. That would be "belt and suspenders". No need for a real heavy wire if the run is short. Connecting to aluminum is always "iffy".
I haven't checked the amps vs. gauge chart but as a SWAG I wouldn't go less than 8 gauge. It has to carry cranking power. Just moving the existing wire is easiest.
Speaking of that here's a table - from off-roaders.com:
