2013 Chevrolet LFX V6 in a 1966 Chevelle- alternator control help needed

I have installed a 2013 Camaro LFX V6 and a 6L50e transmission in a 1966 Chevelle. I now have it running but cannot get the alternator to charge.
I am running the Camaro ECM (E92) but no BCM, and I believe that is where my problem lies.
If I unplug the alternator connector, the alternator starts to charge, but I get a P0621 fault code.
I plug it back in and the fault clears but the alternator stops charging.
Is there a way to have the ECM control the alternator or a way to go around the ECM but not set the MIL?Thanks,
Dennis