Charging problem...attn. electrical gurus

I just finished a new build...65 Chevy II SS...all new wiring / electrical components, 6ga wire from battery to shut off switch to alternator. The wiring system is a Painless kit. The battery is new Optima...the alternator is a new GZ Motorsports, 120 amp racing. The ground from the battery is bolted to the frame and the engine block Here's the problem....the battery has well over 12 volts when tested with a volt meter, but drops to 11.8 with the engine running. The charging system is obviously not working. I'm not ready to blame GZ for sending me a bad alternator yet. The voltage does not increase as I rev the engine. Any ideas, or is the new alternator bad?

Three wire alternator or one wire?
Could alternator ground be compromised by paint on brackets and engine block?

It's a one wire. No paint on block, but the alternator is plated. The only thing I can think of is maybe running a ground strap from a bolt mounting the alternator to the block.

Tried the ground strap....still not charging. I'll call GZ Monday. Has to be a bad alternator.

With the Alternator wire disconnected and running what do you get for voltage. (All loads off)