Alternator help

Lucky you have a 1965. That year and 1963 were the only ones w/ thick buss-bar feed-thrus for the thick ALT and BAT wires in the bulkhead. If they look green on the engine side, unscrew, sand slightly, and connect tight w/ silicone grease. The dash ammeter is often a drop, due to loose or green connectors. There is also a "fused junction" that distributes the ALT feed that often turns green, but unlikely if the car lived in the OC. It is wrapped in vinyl tape in the dash harness. Anyway, you can search for all such voltage drops w/ a multimeter before digging deep. Could be in the key switch, or its connector. As a quick test, clamp a jumper cable from the ALT big stud to BATT+, which will bypass the route thru the cabin.

Other voltage drops people miss is from the alternator case to bracket and engine block. That stopped the alt in my 69 Dart from charging. Also, insure no drop from engine block to BATT-, and from body to BATT-. Note that new cars use many more dedicated ground connections. I guess the designers learned. I am surprised you measured a good connection from the Vreg case to BATT-, given your rusty sheet-metal screws which that relies on.

Re alternators, most people ask the other way about field connections. Most round-backs had one brush (field wire) bolted to the case. All square-backs had two isolated brushes (field terminals). Someone correctly grounded one field in your square-back. You simply need to do the same in your round-back, using a wire. Doesn't matter which terminal. Apparently, you got a rebuilt alternator which was setup for the later style Vreg (isolated field), and they expect people using it on older cars know to ground one terminal. I doubt the 2 thick wires on your alternator's big stud were factory. Where do they go? Factory would be straight to the bulkhead connector's ALT bus-bar. Anyway, hook both the same way to the new alternator. Perhaps one is a bypass of the cabin route (test jumper above), which is termed "The MAD Bypass".