[AzDon]

You are way farther to completion than I am on my 67.... It came with only a broken heater!
So far, I've got a GM R-4 compressor on a serpentine drive with the wires run for the pressure switches and I've got a rear A/C out of an 87 burb mounted. I'm gonna use an underdash heat/air/defrost unit from nostalgic air.com and plumb in the rear a/c.
One suggestion I forgot to make if you are going back to an A-6 is to go to a truck wrecking yard and get all the pulleys and brackets and a core compressor from a 73-87 pickup so that you can depend on GM engineering to keep the belts on.
The reason I like the 73-87 setup is because the compressor is on the drivers side AND you can find this setup under the hood of virtually every one of these trucks.
Compressors pulsate/surge as they run which causes the belt to jiggle and when the compressor is mounted on the upper right, the bottom run of the belt is fed from the crank pulley, so it's the slack run and it is LONG (unless it runs around a smog-pump pulley)
The R4, which replaced the A6, is short and tucks down in front of the right cyl head, so not as long of a slack-belt run.
With a serp setup, the tensioner moves with the pulses, so all of the pulsing problems were solved.
A serp setup with R4 is even better, if you can find one, but your engine may not have all the necessary mounting holes. Additionally, you'd need the reversed water pump and fan and need to convert to the tiny 105 amp alternator.