Won't charge
 

Just picked up a 84 gmc k1500
6.2l with a turbo kit.
This is what I've done and I'm still having problems.
I've replaced the alternator, alternator belt, both batteries the alternator ground.
The alternator and passenger side battery are grounded to the alternator bracket. The drivers side battery are grounded to the a/c pump bracket.
I added a ground that goes from the alternator bracket to the core support and from the core support to the frame.
When I it holds what is left of a charge when I shut it off but as soon as I try to start it it kills the batteries before I can get it started. I don't believe that there is a power drain when it's off.

I'm at a loss! can anyone help?

You may want to remove the starter and get it tested maybe an internal short in the starter itself.