All the bells and whistles are in place for your engine to idle at a really nice 650/700 rpm in gear.
From the sound of it, one of a few things may be the culprit.
Either serious air leak where you suspect, underside of intake, at ports, but this would give a smoke cloud out the exhaust when the engine runs. If this is a small block, there are over 30 different port sizings for intake manifold gaskets, and it is easy to get a gasket that doesn't cover part of the intake to head interface. I've done it a few times when I had no time and didn't really look at the ports/gasket set sent me. Try spraying some WD40 down the PCV valve hole on the valve cover abd see if you can smell it burned out the exhaust, that'd show the leaks inside the valley, if any.
There is also a chance the intake manifold to carb interface is leaking as well, even if the correct gasket is in place there. I have found some Edlebrock stuff to need the same plate the Performer Q-jet base intake needs to use a square flange carb on it. Edelbrock part 2732. It MAYBE needs one of these, do check the gasket for imprinted pattern and any non-overlap intake to carb faces.
Also, try taking your thumb and closing off the PCV valve with the engine running, if it is a problem, the idle vacuum wilol come back up to 15/18 inches and gthe engine will idle down to a nice, normal speed. If this is the problem, there are different weighted valves available, ask for more info here. There is a very slight chance the air volume through the PCV hose is too much, if so, the passage can be ristricted with a slug, made from the unthreaded section cut from a bolt, with an .050 hole drilled through it, then installed in the carb end of the hose.
More likely, cam is retarded a tooth or the valves/lifters are too tight. If the engine doesn't miss, and idles fairly steady, I/d go for the cam being off. One thing that will kinda sorta help the idle with retarded cam is MORE timing, lots more timing, and it won't hurt starting as one would suspect and see if it were way too far advanced. If the thing will still start OK with upwards of 25 drgrees BTDC, I'd go the cam, definately.
Retarded cam will also give very poor idle & low/midrange power. but fair top end power, not great but fair.
After confirming all other vacuum related issues are in fact OK, I think I'd last ditch effort recheck the valves, engine running method is fine, you know how, back rocker off, tighten 1/4 turn past noise going away (hydraulic cam), etc. Then, if it still did the stuff its doing now, go for the cam.
Please post what you find and we'll go from there.
[This message has been edited by IgnitionMan (edited 08-10-2000).]