Bob I agree with ya crankcase vacuum is a good thing, but 5" is alot, it has to have a source creating it. The only source that can create that much vacuum is the induction system. Hence my statement " you have an internal vacuum leak causing high CRANKCASE vacuum".
My thought is that you may have an intake manifold vacuum leaking into the crankcase.
We can throw all of this out the window if you are running a vacuum pump and or a header e-vac system or just call me Nuts.
Just for conversation how much intake manifold vacuum do you have at idle? What is your current idle speed?