Alternator upgrade charging problem

My 1965 with a dauntless v6 had an origional 37 amp external regulator working fine. After installing a sound system, HEI distributor and a winch I upgraded to a 94 amp delco 12si 3 wire off a 1986 chevy pickup rebuilt donor engine. I had it tested at auto zone and it passed, I installed it with the positive from the battery to the starter to the batt on alternator along with the hot wire to the ignition. The ground from the alternator to the same bolt as the battery ground on the engine block. And the idiot light wire which is hot with the key switched to run position to the 1 spade terminal. The battery tested with a voltmeter has 12.6v with the engine off but with the engine running is below 12. I checked all connections are fine so I brought it back to a different autozone and it still passed. Am I missing something here? Thanks

 

Do you have a wire from Bat terminal to the #2 spade terminal? Gotta have that as well.

 

Thanks that is what I am missing