Seriously another alternator question.

Just to add to the confusion; somewhere between 1972 and 1976 the field current changed from 2.5 amps to ~ 5 amps. Your alternator was probably rated 30 to 35 amps. To put 68 amps thru the firewall terminals when the battery is about dead is a real bad idea. Adding a 'bypass wire' from the alternator stud to the starter relay big stud will help/cure that. Use a # 12 wire with a # 16 fusible link. Your ammeter will read about half current.

This won't cure the 15v at idle problem. Look for a poor connection. See Dart 273 above.