Cruise Seems Lean

Eric,
Been thinking about this and there are two paths you could take.
1. Fix the radiator, put in new plugs, connect the distributor as before and retest the situation.
2. Install primary PVs and/or adjust the distributor set up, and start from (nearly) scratch.

Part of that decision should be based on how much time and effort you want to put in at this time.
Whatever the setup has been was pretty close to making it usable for your purposes.

Part of that decision would depend on knowing a little more about the set up as its been.
My suggestion is to gather as much information as you can about the setup. Lets call it 2020 since its was unchanged from last year.

I agree that a vacuum gage would be helpful. I'll add to that a vacuum pump, and assume you have a tach and timing light.
It would be really helpful to know more about the 2020 timing. Part of that is whether the manifold vacuum was actually moving the advance at idle.
Your 660s look like older units. 1970s or early 80s unless someone was swapping parts.
My guess is the base plate vacuum ports are live. If you have the carbs off, look underneath and you can see the connection.

The next question is whether having the vacuum advance connected to that port made a difference. If you know the vac at idle and the vac required to move the distributor's advance plate, we'll have a better picture as to what was going on. Then we'll know if you make a change to ported how radical that change will be.
What distributor?