In my opinion, it was marginal still for those days standards.
AGREED 
About the wire between alt and batt, that will VIRTUALLY bypass the amm but, also provides an extra path for the load coming/going through all the network. If some problem is present on wiring you are
not fixing but hiding it. AGREEDThe OP can be confronting several issues at the same time, but yes, still with an inoperative alt ( for WHATEVER reason ) the batt should supply the power to still keep running the engine. So it could be a problem between batt and main splice ( ammeter included ) down the under dash harness tape which spreads the power to everywhere.
thank you for responding
My suggestion to run a jumper from the battery to Alternator was to prove the real issue was between the battery and alternator feed and that the car should stay running even with low or no output from the alternator for reasonable periods of time>
If the OP is wanting to verify,
Disconnecting the field wire(s) will shut the alternator down and should cause the vehicle to die that would prove what we are saying.
if the vehicle does stop running, running the suggested jumper from the battery would PROVE that the connection issue is being hidden by the alternator.
thank you again
