I did the ammeter bypass recently and it required no other modifications. I simply removed the connectors from both sides of the gauge and connected them directly together.
The benefit is that you won't have to carry the entire current load of the system through the firewall connector. As...
If you're not replacing the wire, the 12g should be fine. However, if you're replacing wire, I'd go with the 10g. Whenever I replace wire, I bump up the size to the next gauge. I'll be watching this thread for other opinions!
Mike
My car was recently having a similar problem, charging system swinging wildly between 13 and 17+ volts. I ran a new wire from the battery positive terminal to the blue (voltage sensing) wire on the regulator and a new wire from the field on the alternator to the green (ground) wire on the...