It is sounding more and more like an intake gasket failure. Joe-JDC
Joe, you know it's probably an intake leak. But, now you can say that a vacuum gauge doesn't always detect intake leaks. At least mine didn't.
I would add that an intake gasket leak RARELY shows on a gauge. If you think of the volume of a couple/three pistons dropping in sequence, and the size of the intake plenum, a small leak will likely not show on the total vacuum shown on the gauge, although it will change a/f mixture and add oil dilution for that specific cylinder. Matter of fact I am considering unhooking a PCV on the truck I am working on tomorrow (396 FE) to see how much the vac gauge changes. That'll be a 3/8 hose off #4 port, so it should be dramatic compared to a seepy gasket:)
Your problem sounds like an intake leak.
BTW, I saw you mention umbrella seals, get their perceived success out of your head
They were oil deflectors, not seals, they did OK compared to Chevy O-rings, and for long term, they likely outlast Teflon seal (but not outseal at the beginning, regardless I hate those hard plastic seals), but compared to modern positively located Viton, unbrellas are not very good.