I read your other post. Maybe should try to keep post together.
To the problem. Your alt is showing a charge when you rev it. At idle with the A/C on it drops to about 11v. I think your alt is working fine.
I would check your voltage regulator. Even if it's been replaced before!
Believe me when I say, replace it even if it has been replaced already.
Edit: I like to use the later regulator for my 67. part: C8TF-10316-A its rated for 55A It's a relay type. Large housing. Quit using the full electronic ones, after a brand new one, gave up after two months. Scratched my head on that one for over two weeks.