Alternator help

I have a 65 valiant slant 6. Charging system was upgraded to a square back alternator with electronic regulator before I owned it.
Lately I've been having some odd electrical problems. Radio shuts on and off when headlights turn signal are on durING idle and sometimes at highway speed. Checked with multi meter. Idle I get 12.25v at the alternator. When I Rev the pissue out of it, it barely reaches 13v.
I have a spare alternator that was in a bunch of slant 6 parts I picked up. This is a round back with 2 FLD prongs that have continuity to each other but not the through the housing.
How would I set this up if it will work? A buddy of mine says he's never seen one like this. Could I just hook the 1 FLD wire to one prong and the 2 thicker wires to the large stud? I have no idea if this alternator works to begin with but might as well test it out