no headlights or anything+upgrade alt question

I rewired my 74 Duster with a complete harness from a 74 swinger-inside and out. Even took steering column from swinger/valiant. Anyway- dumb question first. I dont have the ammeter hooked up in dash yet. Would this cause me to not have any juice inside car and to all lights inside and out? From what I can see- looks like it may be the direct electrical connection for everything-the ammeter. Is this right?

Second question. I installed an alt from a 79 Cordoba-100 amp. Just nice to have the extra output. Do I need 4 gauge wiring for this? Sounds really thick compared to what I remember being in the Cordoba?? Will this 100 amp fry my ammeter? If so- how do I bypass the ammeter to run dash and lights-if 1st question is because of ammeter?

I dont have a key for the steering column-hopefully I can get that switch out without too much problem.
I have juice at the starter relay under hood, but seems like thats the only place. It turns over. No gas in car yet.