(so many Marks on the bulletin board
) 5" Hg vacuum at idle is pretty low. I'm suspecting you don't have enough static ignition advance. Or at least that's the first thing to check.I'm told the 3V heads require only about 30 degrees total ignition advance, which is about 6 degrees less than the iron heads. I don't know if that holds true at idle as well. An iron head with a cam like yours can run around 20 degrees static advance (with no vacuum advance, or with the vacuum advance connected to ported vacuum).
You can optimize the static setting in the driveway. With the engine up to temp and idling, vacuum advance connected normally, slowly turn the distributor in the advance direction (clockwise?). As you turn the distributor, the idle speed should increase, continue to turn the distributor until you just reach the point where the idle speed stops increasing, this is the ideal setting for the distributor at idle. The intake manifold vacuum will be at it's highest point, the exhaust temps will be at their lowest temps, you'll have more low end torque, better drivability, less heat load on the cooling system, and a better idle.
However, that will probably throw your total advance setting way off, ideal total advance can only be determined on a dyno.
If you arrive at 20 degrees static advance using the method I just described, and your engine only requires 30 degrees total advance at around 3000 rpm, then your distributor should only have 10 degrees centrifugal swing. Most distributors are set up with about 20 degrees of centrifugal advance.
After you determine the optimum static setting, and the optimum total advance (a trip to the dyno is required) then you can have your distributor recurved for the proper amount of centrifugal advance.
You can check for vacuum leaks with a can of starter fluid. With the engine idling, spray the starter fluid wherever you suspect there may be a leak, if there is a leak the starter fluid will make the engine speed increase.
your friend on the DTBB, George