Need help with this vacuum reading

I had a 360 with low compression and a big cam. I had a problem very similar to yours and I was able to solve it. At idle I put a timing light on it and had someone else put the car in gear. What happened to me was that it dropped so much rpm when I put it in gear that the distributor pulled out timing due to the centrifigal advance. My car had a Mopar Performance electronic. What I did to solve my issue was lock out the centrifugal advance and it idled nicely but still had only 5-6 inches of vacuum. The low vacuum was due to a combo of an old 509 purpleshaft and 8 to 1 compression. I bought an MSD billet distributor and played with different advance curves and finally just locked it out. My motor, since it had so much overlap, was able to run on 87 octane without detonation. I hope this helps you out. After seeing your video it really reminded me of my problem from years ago.