Not knowing what the original output VS. the new alt output it is hard to say what might happen or has happened.
If your new alternator is still running through the old amp meter and old wiring to charge the battery and provide current for loads you can have serious issues.
First to fail is the weak point of the whole circuit the fine wire that moves the needle in the amp gauge itself. This would be seen by no needle movement.
If the current is to much higher then original then the next to fail would be the old main wire or the shunt in the amp meter. This would result in draining the battery till out of juice.
If you want to keep the vintage look then the old amp meter needs the new shunt to handle the extra current plus a re-lable on the + - current level on the meter face.
If you desire a volt meter in place of the vintage amp meter then the amp meter needs removed with the two leads removed from it tied together to bypass the amp meter. These could be placed on the positive post of the volt meter to provide its source for sensing.
Or in today's age of digital an amp/voltage meter all in one could be placed in the cab to view both amps and volts on one display.
No mater what your choice, determine old VS. new currents. Do this to see if new main wire is needed and what is needed for an amp gauge.
