Alternator toasted - Advice needed please

The car is a 1971 Swinger with a 340 now instead of the original 225. I recently finished a complete rebuild of the entire car, pretty much everything except exterior paint. The original wiring harnesses were butchered by the PO so I replaced everything I could buy with new M&H harnesses, including the dash, forward light, engine, heater/AC and rear lamp harnesses. Both battery cables are new repops as is the VR. I also replaced the alternator with an OEM style 3-wire rebuilt unit.

With some amount of anxiety because of all the new parts I hooked up the battery cables yesterday. Initially I was encouraged as all the lights and interior accessories worked, including the key buzzer, etc. In other words, the circuits "hot" in the key-off position were ok.

When I turned the ignition key to the run position the alternator started smoking and before I could get back inside and turn off the key the alternator looked pretty cooked inside. I checked the field wires and the 12V blue wire had some bubbled insulation within the first inch of the terminal but the rest of the length was ok. I checked continuity of this blue wire back to the bulkhead connector and it was still intact and showed no signs of overheating anywhere but that first inch. As I said, the wiring inside the alternator looked blackened.

The alternator did have the nylon insulators under the two field wires installed and they looked ok.

I have another alternator and am about to try again. I am hoping, maybe naively, that the alternator itself may have been the trouble. I read through some earlier forum posts on this topic and have since made sure my VR is grounded by scrapping off the paint under the mounting bolts and reinstalling the bolts. I will make sure the field posts on the new alternator are not grounded before I hook up again.

I would welcome advice on what else I can do to avoid a repeat of the problem before I hook up the battery again. I need the "for dummies" version as electrical system debugging is not my strong point.