Power Disc Brakes not working on hard stop?

Just a data point. I have had a 67 barracuda 360 4-speed with manual stock KH discs and stock 8 3/4 drums. Stopping was good from all speeds.

Question, because I don't know the answer: If you disconnect and plug the vacuum line, can the brakes be worse than manual? If not, you could try that and see if the high speed stopping problem is still there. Based on my good experience with manual disc front brakes, your leg force should be more than enough.