SJSS, I was fighting a odd idle condition also and here is what I found.
I have a very similar engine combo
60103 luanti cam
rhs heads 180cc runners 64 cc chambers
600cfm edle carb
9.48 to 1 compression
mallory hei distr.
With the engine idling at 1000rpm vacuum is 14" at the intake manifold. If I lower the idle speed the vac drops. At 750rpm the vac is aprox 12". The vac canister starts to hunt at this level, meaning it does not have enough vacuum to keep the canister fully engaged. I can feel the vac can pulsing at lower rpms which is directly connected to the distributor advance plate, therefore causeing the timing to advance and retard very fast. Not good. I can live with with a idle speed of 1000rpm but with a auto trans that puts the idle at 750rpm while in gear which is right where the vac can starts to drop out. I beleive crane makes a adjustable vac can that will operate at lower vacuum but havent looked into that area yet.
Currently this engine is set at 24deg initall, 12 deg mechanical advance and 12 deg advance on the adjustable vac can (which is stock from Mallory). I was running the vac advance on full manifold vacuum port. Yesterday I hooked the vac hose to the ported vac connector on the edle carb and now the idle is solid and steady. More testing is on line for today but honestly I have been fighting this odd idle problem since the new engine was installed in march of this year. All this time I've been searching for a electrical problem causing the miss at idle but I'm very confident it is the vac advance system causing the trouble. Hope this helps.