With the indicated items you have replaced, the only thing remaining is the wiring. Other comments here are good suggestions to narrow down the problems. If you inspect your related wires and see any greenish corrosion looking into the housing where a wire enters a connector or where a wire is crimped on a lug, that's a potential problem. With only a nominal 12 volt system, even a slight amount of resistance in any of the wiring from poor or corroded connections will be problematic.
Concerning grounds, if you wiring is as it would be if original, you should have a good solid heavy wire connection from battery neg. to front drivers side of the engine block. On passenger side rear of the engine block, you should have another heavy wire that provides a good solid ground from the engine block to the firewall. All with good crimps, no corrosion and tight mechanical connections.
For the Voltage regulator, I would suggest making a short ground wire with proper sized lugs to tie the regulators case to that firewall ground, or assure that there is a good solid clean ground connection to the bulkhead from the case of the regulator.
As a previous posts indicated, the heavy + 12 volt wiring that goes thru the bulkhead connector MUST have good solid electrical connections, no corrosion and good crimps. Same for the inside of the bulkhead connector under dash. The wiring to / from the alt gauge is critical. Good solid crimps and assurance that the mechanical connections to the studs of the Alt gauge MUST be solid and tight. It does not take much resistance in the main 12 V electrical path to cause heating at the connections. Once the heating starts, and just continue to get worse with time till you have a meltdown or worse a fire. The fire extinguisher is a great idea in a car as old as this. I would sugest a Halon or CO 2 type, not a dry chemical type. Good luck with your issues..