'68 Barracuda - charging system questions

Seems you have t.m.i. already but I'll add what I know. 12 volts goes into ignition switch. At switch on 12 volts goes out on a couple of wires. One goes to fuse box. Can we assume you are getting 12 volts to radio, wipers, signals, etc..? Another of the terminals on your ign' switch has 2 blue wires going out of it. One of those wires goes to inst' panel to feed inst' voltage limiter. Other goes through bulkhead to engine bay. It feeds 12 volts to system voltage regulator, etc...
So there are very few places where there would be a weak connection/voltage drop, the ign' switch, the wire itself, the bulkhead terminals.
Bypassing the amp gauge in the engine bay renders the gauge wrong, pretty much useless. May as well take it out of the circuit/current path but... both wires onto one amp gauge stud is not the best method in the case of rally inst' panel. I won't go into the "why not?" here except to say that entire gauge is still hot if the wires are on it.
Optimal solution is replace amp gauge with a volts gauge. Put red and black amp gauge wires together with crimp connector, solder, heat shrink. Volt gauge would connect to switched 12 volts, the blue wire that feeds inst' voltage limiter, or the wire that feeds park brake lamp ( I forget what color it is ).