New to Tuning, How to read vacuum

I agree with the assesment Fishermans67 provided.
I don't have any info about the cam or specs for a '76 RV 360 but can specualte it pulled hard down low. While it probably had some tweaking of the exhaust lobes for emmissions (mentioned in the early 70s Master Tech booklets) a timing curve that looks something like MP used will probably be a decent starting point.
Even though it seemed to like 14* in neutral, it probably will actually be stronger in gear with a little less.
When you get the tach, after its warmed up, see if it will idle decently with 12* around 700 rpm.
If you find its at 14* but 1000 rpm, leave the timing alone because it will drop when you reduce the rpms. Then remeasure.
Only if you find its at 14* around 600 to 750 rpm, then try a couple degrees less with a slightler richer idle mix.
It will take several tries going back and forth between adjusting the idle speed screw downward (counterclockwise) and the mix screws.
When turning the mix screws inward (clockwise) there should be a point where there is noticible drop in rpm. Turn back a fraction so the rpm recovers, then turn them 1/8 to 1/4 turn richer. Then check in gear. The least drop in either rpm or vac indicates the strongest idle.
If turning the idle mix screws in doesn't cause the engine to die, then the throttles are too far open.

Once you get the idle good, taking it for a short gentle drive to test is OK. Do not floor it. (We know nothing about the curve in the distributor, so moderate load, and especially full load, situations need to be approached with caution.)