idle questions

You can probe for a vacuum leak w/ propane. It should smooth out when you flow it near the leak. Never worked for me, but I probably didn't have a leak either.

Your compression numbers are a bit low, but don't get excited since many ways to read compression wrong. Turn the engine over by hand by tugging on the fan belts and verify you must fight 3 strong air springs per revolution. If so, the compression is fine. However, interesting that your clean plug is in the lowest cylinder. I had a bad cyl in my 69 slant that just flowed raw gas, so it was perfectly clean inside. Your spark plug looks like that.

Still, low compression or a dead cylinder wouldn't stop your car from idling faster. Not sure why it will only idle 450 rpm max. I would expect the other way - "won't idle below ___ rpm without dying". Is that with the idle stop screw on the linkage all the way in? If so, you aren't getting enough fuel, so suspect the carburetor. What model is yours? The Holley 1920 was bad for me (3 carbs) until I found a great one.