New to Tuning, How to read vacuum

Projectile Dart said
Carb is a Holley 3310-1 with 66 primary jets and 72 secondary.

This is an aside and has nothing to do with the setting up the initial timing and idle/off idle that we've been focusing on.
3310-1 came with 72 primary main jet (PMJ) and 76 secondary main jets. 10.5 Power Valve in the primary and 8.5 PV in the secondary.
Looking down the carb you may notice tabs sticking out from some of the boosters. If so, those are to direct fuel for the most even distribution on a factory 396 cid Chevy (such as came in a '65 Chevelle).
Anyway, if the 66 PMJs are too restrictive, the engine will surge and try to die when cruising at interstate speeds. If so, go up at least two sizes.

Projectile Dart said
It has been running far too rich and I've noticed a "hiccup" or "stutter" at part throttle cruise and the car has been running far too rich.

Your RV 360 has stronger (higher) vacuum at idle than that hot chevy engine. So what may be happening is the stronger vacuum is resulting in more fuel for the same amount of air flow in low throttle positions. The 'idle' circuit supplies fuel at all low throttle positions. Cruising at 35 mph or 45 mph take very little power and so a low throttle position.