What are the specs of your "squareback" alternator presently in service? What is the drive sheave diameter? Is it a 2 groove sheave? Will this alternator produce 14.5 nominal volts (+/-)? IF it produces the correct voltage, it is possible that it has an OPEN diode, which exhibits OK voltage but under load cannot produce the necessary amps, which results in lower voltage. As I've said numerous times, the MOPAR alternator is a three phase (3 phase) full wave bridge design that uses 3 positive diodes and 3 negative diodes. AN OPEN DIODE, either positive or negative type, will result in one half of the alternators output (low voltage and insufficient current)......it is possible that this is the case. The alternator must be disassembled to test individual diodes, they cannot be group tested....in spite of what the auto parts person tells you. If they take it in the back to "load" test it, inquire about the test and you want to witness the test against a published test procedure....if they cannot provide a test report or allow you to witness the test, they're just blowing smoke up your ***.......just my opinion.....
BOB RENTON