Dear Sam;
They (both) only put out the current required to maintain the voltage (demands) of the system.
All vehicles today have an A/C option when new and why we have Alternators. Without A/C a Generator is all you need for a 1948 vehicle.
Even with all the lights (& high beams) on, all those circuits pull is around 20 @ 6 volts and less at Twelve.
The exception being;
Say you run out of gas and have to crank it a lot for restart; now your battery is lower than 'normal'. For a minute (or so) this 60 Amp. unit sees a voltage demand and cranks out 50+ amps to make it up. A 30A 1948 ammeter can't handle it and fries. Now; If it 'shorts' in the process it toasts the wireing too; as Bill has mentioned.
Voltmeters are used today as they are NOT in 'series' with the charging circuit and this can no longer happen, which Walt has addressed.
Happy trails.
