Low Vacuum? With good compression?

[MagmaJct]

I've had my truck for 7 years now, got it from my Uncle after he passed away. (350 SBC) As long as I've known, the idle vacuum has been low. ~10 in/mg. This has never really caused a problem, the engine idles fine with the Carter AFB. I get 10 MPG, empty, loaded, uphill, downhill, and it passes emissions.

I've decided to replace the carb with a Quadrajet 4MV. But, as you could imagine, it does not like low vacuum. The power piston is staying down, however, to maintain idle speed I have to open the throttle enough to cause main nozzle drip. I've verified the float at 1/4". (It is brass, however.)

Instead of trying to make the 4MV work, I'd rather address the low vacuum. I just completed a compression test: Range, 160 - 175 PSI. All pressures came up within 5 cycles.

I've replaced carburetor and intake gaskets in an attempt to correct potential intake leaks. None were found, no improvement. I'm not surprised, any vacuum leak that would drop the vacuum that low would cause other problems, like high idle, lean idle, and misfires.

What's left?
1) Late valve timing.
2) Uncle put a performance cam on it.
3) Intake leak from exhaust crossover.
4) Anything I missed?

How do I verify? I'm a family man, and would prefer avoiding pulling the harmonic dampener and timing cover if I can, due to limitation of my free time.

I'm considering investing in a leak down tester. It may prove useful.