Pictures of both AMP gauge fuse locations?

[Accelo]

Changing the alt. wireing will have no effect on the ammeter. "

Maybe not. However, typically installing a one wire alternator is for extra capacity. It is installed for options like electric fans or power seats. You should not be running 100A through a 12 gauge wire.
The old school amp gauge is run on the "shunt principle". Both sides of the factory amp gauge have 12V. The amp gauge is, in reality, a micro volt gauge that is calibrated in amps.

Because of the low voltage difference between the connections they have to be perfect. I have repaired many a amp gauge by cleaning the terminals. Starting with the connection at the small black terminal and continuing to the connections at both fuses.
The system is temperamental because of this and is one of the reasons GM discontinued the shut amp gauge in favor of the voltage measurement.