Hey Drew!
I am sorry you have to get hit with one of these "problem children" to start your career as a Gadgetman. But it is what it is... Perhaps you can connect with one of the family. We'll see if anyone offers, but if not, you're going to have to solve this one yourself.
With a manifold vac of only 17 at idle, that is a statement of poor integrity of your manifold. Somewhere, something's leaking. It may be almost anywhere, so you have to practice the process of elimination.
I start with the largest hoses and check them. They would be the Booster and the IAC. As you have already done the PCV (are there TWO vac lines to this? CAP THEM BOTH@ Ford is the only MFGR that uses two-SOMETIMES-of which I am aware and you gotta get them both.) then move on from there.
On the brake booster, just put your vac pump on the hose at the intake manifold. Start ALL your testing of sub-systems there and when you find a leak, trace it back to source and correct it.
On the IAC, here's a good way to see if it's closing. Remove it from the intake. Two 8mm bolts hold it in place. Cut a new gasket (you can use a business card), opening only ONE of the ports for air and reinstall it.
The engine will stumble, no doubt, but with the port closed the manifold vac should be in the range of 19" hg.
There is one other suspect connection. That is in the line that feeds the climate control for the cab. FREQUENTLY leaks develop there and no one catches them. IMHO, these are the 'silent killers' of your MPG.
So, get ready! You may find the leak right away, and you may spend a few weeks on it as you explore.
The LAST place to look at is the manifold gasket itself. The Chevy 305 and 350 series engines are notorious for leaks from exhaust gas passage directly into the intake ports on the heads. Invisible and impossible to diagnose, when all other options have been exhausted, replace the gaskets.
Several have done this and found another 20% MPG gains hiding there, so it is something to look at, even if you're already happy!