the alternator will not put out 100 amps [or even 50] unless the battery and accessories combined ask for it, an original 54 generator would probably have been rated about 35 amps, so likely about 30 is tops a stock truck would ever ask for, although there can be situations just after starting there might be a momentary spike
if you want to be totally safe, replace the main output wire from the alt to the battery with 10 gauge, and yes, replace the ammeter with a volts meter or put a shunt on the ammeter so it doesn't take full current
Bill
