Easiest upgrade on a slant six
Use a Denso alternator from a 1988 Dodge truck.
Approx 100 amps and dual v-belt pulleys. If you use the cast slant six alt bracket, just cut 1/4 inch off the front of the bracket, and eveything is a bolt in swap. Even the selectrical hookup, including the stock Mopar late model voltage regulator. BUt I would upgrade the wiring being fed by this (or any high output) alternator, or you risk having and electrical meltdown.