1967 Pontiac Lemans Idle Problem

I have a 1967 Pontiac Lemans with a 326. The engine was completely rebuilt last year with a few modifications from stock. I am running a Holley 4160 600 CFM 4BBL carb on an Edelbrock Performer Series dual plane intake manifold. It’s got Pontiac RAM Air cast exhaust headers (like came on some GTO’s on the same vintage). It’s running a Crane Cam (#283902 which is a mildly more aggressive grind for RV, light towing, off-road usage that should be smooth idling and more economical). The heads were completely rebuilt replacing everything including switching to threaded rocker studs and adjustable push rod guides. The engine was also sent out for complete balancing. I’m running Gibbs Racing Oil with high zinc content and premium pump gasoline.

At any RPM above idle the car runs great. It’s getting about 14 MPG combination around town and highway which is probably a given in light of vehicle weight, 2.30 rear end and the 2 speed Super Turbine 300 transmission.

My problem is at idle. It’s rough, occasionally fluctuates (lopes), and is generally unresponsive to tuning efforts. The best vacuum reading I can get out of it is 13 in HG and it seems to like an idle speed of about 650 RPM which is higher than factory spec for an automatic transmission equipped car. The vacuum gauge is also vibrating a bit and not steady. I have adjusted the idle mixture in minute increments up and down with little change in performance or vacuum level. I have searched high and low for a vacuum leak without success. I’m going to take it into a shop with a smoke tester to see if we can find a vacuum leak by pressuring it up with a the smoke pump. The consistently low manifold vacuum keeps me thinking either vacuum leak or maybe a valve timing issue.

Any other thoughts?

Thanks