Alternator gauge is smoking

The ammeter circuit is a major circuit, and the ammeter is in SERIES with all loads in the car except the starter. It is therefore, a "heavy" circuit and IT IS NOT FUSED except for the main fuse link, which is poor protection

It could smoke at any time as it is always "live." You are saying "only when running" which implies probly a charging problem, as the alternator increases current

Make CERTAIN the ammeter is assembled correctly and that the studs/ nuts are tight. If so, check charging/ running voltage to make certain the system is not grossly over-charging, perhaps a wiring error, something wrong with the regulator or the alternator

72 has the "isolated field," often INcorrectly called "dual field." IT is common for one of the alternator field terminals to short to ground, and if the blue wire (switched 12V) is hooked to the opposite alternator field terminal, the alternator will charge "full steam."

I WOULD NOT LEAVE THE VEHICLE with the battery connected until you get this problem "found"