Put a timing light on it and make sure the timing is rock solid in and out of gear. I'm suspecting that at 1300rpm's you are off the idle circuit and some of the mechanical advance is in as well. This will make idle tuning difficult if not near impossible. Idle speed should be down around 650-800 rpms, not up at 1100-1300.
The cam is small and poorly chosen for the big 455HO so it's going to give you some grief at idle as well. You are seeing some of that when it "skips" when the vacuum advance is hooked up and timing jumps to 30 degrees.
I would do two things before continuing. Check the distributor springs and mechanical advance to make sure they are strong enough to keep the timing from advancing at idle speed.
Reduce how much timing the VA adds back to about 12-14 degrees and start over with the tuning.
You never mentioned total advance or all-in w/o the vacuum advance, but I'd start around 30-32 degrees and work from there.......
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73 Ventura, SOLD 455, 3740lbs, 11.30's at 120mph, 1977 Pontiac Q-jet, HO intake, HEI, 10" converter, 3.42 gears, DOT's, 7.20's at 96mph and still WAY under the roll bar rule. Best ET to date 7.18 at 97MPH (1/8th mile),
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