I found that the bulb for the ammeter got way too hot (bulb furthest to left) . Boiled the plastic lens (also pictured). Could something as minuscule as a bulb cause a short like this?
That bulb IS fused and with a quite small fuse, the "INST" fuse at one end of the fuse panel, PLUS the illumination lamps are all controlled by the "twist to dim" control on the headlight switch. So, no
Whether you bypass the ammeter or not--and there other methods of doing so--this article gives and overview of the what/ how of ammeter circuit failures. Even "Ma" knew this as Chrysler started doing away with the "full current" ammeters in the big boats, and finally in A bodies in either 75 or 76. Also, on the cars WITH "full current" ammeters, Ma did a "partial" bypass of the bulkhead connector on optional 65A equipped cars. This is known as "police fleet taxi" wiring and is likely in your manual for maybe B bodies.
What can happen "right at" the ammeter is that the studs get loose and generate heat and then from there self destruct. Also the wire terminals can fail and do same thing. Likewise, the terminals in the bulkhead connector were NEVER meant for the kind of current they must carry
When you installed the integral regulated alternator, what/ how did you do/ wire up the green and blue field wires off the old one? I'm thinking THAT is the start of the problem