Sounds like there is 2 inches between you and happiness, hehe..........
Even with all those miles, you should be able to get 17"/mercury on a stock engine, unless you have a bad cylinder , early/late timing problem, vacuum leak or checking the vacuum at an unreliable source ...............
Other factors also contribute to low manifold vacuum but, all in all, that motor should be pulling at least 20"/mercury at idle........................
With a little effort I'm sure you'll find the problem......................
UNCLE PETEY.....................