Guys had cautioned me about using propane or carb cleaner to test for a vacuum leak simply because the stuff can easily be sucked into the intake and basically give a false positive indication as to the presence of a vacuum leak.
Being one to resort to the simplest home-grown solutions to complex problems, I came up with what my buddy dubbed the "Benjamin Moore vacuum leak tester".
The 4.3 V6 has a collar that sits atop the throttle body and the air cleaner in turn, sits on the collar. The collar also has a hose nipple on it where the fresh air hose for the crank case vent air intake attaches.
I took a gallon paint bucket ( I don't really know if it was Benjamin Moore paint or not) and cut a hole in the lid that was the same diameter as my air cleaner base. Then I placed the collar on the throttle body, and the inverted paint bucket on the collar and ran a hose off of the nipple on the collar down the left front fender and away from the engine in general. Now with the engine running and breathing through it's new snorkel, I could dispense the carb cleaner around the base of the throttle body and along the sides of the intake manifold where it meets the cylinder heads without fear of feeding the stuff into the throttle body and sure enough, I could make the engine change RPM with the carb cleaner. That was enough to make me decide to change the intake manifold gasket.
When removing the manifold, I couldn't find any evidence of a a vacuum leak but vacuum leaks are sneaky and they hide real well. I was hoping for something unmistakable but it wasn't there. After we finished the gasket change, we fired the engine and it had a great deal of trouble staying running during the first few minutes but once it was up to temperature it ran really well and the idle RPM was much more like what I would consider a normal speed. The exhaust note was also steady with no pops or surging. I plan to double check the timing and do the minimum air/base idle speed set up again soon.