Heater core open with AC running

[HBlue]

I'm working on a 76 Custom Deluxe. Part of the restoration was replacing the AC compressor that had blown. When I got it back, it just didn't seem to be putting out air as cold as I expected. I took part of the duct work off and found the lower right side of it was hot. My heater core shutoff valve must not be closing completely. I adjusted the shut off but that did not fix the problem.

I once had a 72 Bronco that shut off the incoming water with an inline valve. Can you still get those or is there another solution I'm missing.