Alternator upgrade

As for the max amount of current your amp gauge or more importantly the firewall connection can handle, that depends entirely on the quality of the connection. As these connections age and corrode, they develop internal resistance. The more current you try to stuff through the resistance in the connection, the hotter it gets. the more it overheats the more the resistance goes up, and pretty soon the whole shebang degrades to the point the battery wont chanrge and you wind up on the side of the road with a dead battery and a melted down bulkhead connector (if you are lucky and the car doesn't catch fire).

Easy solution is to run an 8 or 10 AWG wire from the alternator directly to the battery cable connection on the starter relay. Now the entire charging load goes directly to the battery bypassing the bulkhead connector. The amp gauge will only show the amount of current the primary circuit draws. A central binding post for all DC connections like superdart suggested works well with a remote battery setup, and would be the ultimate solution.