The only thing fed by the negative terminal on the coil is the points & maybe a condenser. Otherwise you run the risk of burning your coil out by pulling too many amps through it.
If the alternator needs a field "exciting" feed, take it from the ignition switch. If you take it from the positive terminal on the coil, you may overload the wire from the ignition switch to the coil as you would be feeding two items through a wire that is supposed to only feed one, the coil.
I personally like to have both the ammeter & a volt meter. I have not had any issues with ammeters. IME, they are not, "one of the biggest causes of wiring fires" If your wiring is in good shape & the gauge is installed properly it is not an issue. On the other hand if your wiring is in bad shape, +/or the ammeter is installed incorrectly, It can be an issue.
The same is true of voltmeters, but the wiring for a volt meter is small enough that it will fry quickly & usually not start a fire.
As far as having "Full voltage present full time". Even if you eliminate the ammeter, you will have "Full voltage present full time", at both the ignition switch & the headlight switch. Not to mention the horn & the brake lights. If you are worried about this, install a battery disconnect in between the battery & the switches.
The original wiring system worked well for over 50 years. I would make sure that the wiring is in good shape. If I have any questions about the condition of the harness, I buy a new one. It is cheap insurance & has the added benefit of no shorts or bad connections to chase.
I would remove the generator regulator from the electrical system. This will eliminate any "back feeding" that may be happening with the alternator. Again, less wiring to cause you grief.
IHTH, Cpt Logger.