I'm travelling right now and can't post as much I'd like due the poor connection. Was trying to post this since couple of hours LOL. Later will get deep into some sugestions.

I'm noticing also you are talking about the shunted system! Need to say the shunted system is different from the full load ammeters. Shunted system or wiring is a remote ammeter reading using miliamperes running throught the 16 gauge wires. You won't get same results wiring a full load system like a shunted systems. Ammeters with shunted systems gets a mounted on coil to amplify the needle movement with just miliamperes thanks to the resistance the shunt provides.

Attached an ammeter to shunted systems, used on Mopars since 75/76 so be careful what diagram you use as a guide!

Ammeter for shunted wiring