Since you went into details, I would bet the farm that it is indeed, a vacuum leak. My car had the exact same problems. Not similar probs but, the exact same problems. An easy way to check if the car has a vacuum leak is to squirt/spray some ether or starting fluid around the intake and base of the carb. If the car picks up idle when you spray it you will know for sure that it is a vacuum leak. Did you guys put a spacer under the carb? Did you check to see if it was "true." I had a leak there with my cheap Summit phenolic spacer. If the spacer is wet it too would be considered a vacuum leak
Again, I went through the same thing with my car. It was running rich, fluctuating idle, ran great at higher RPM´s but the plugs were drowning. BTW, how do the plugs look? Set the timing to 10 BTDC and 36 total advance. Another thing is that the TACH on my car was jumping all over the place. Then we thought that the carb was too big for the motor since GM recommends the 600CFM and we had put a 750cfm on it. Changed jets (down), advanced the timing, retarded the timing, fattened the mixture, leaned it out and then out of curiosity we checked for vacuum leaks and BAMM! there it was right on the back of the intake which in turn screwed the intake runner gaskets. So, we changed the intake gaskets, put the original #72 jets back in the 750cfm and set the timing again at 10 BTDC and 36 total advance and the car now runs like a champ.
Just humor me and check it for vacuum leaks. If I´m wrong I´ll buy you both a cold one
When I read the original post I thought that I had written it since it was so similar to my problem. I could be wrong but as you stated, you have to check the easy most obvious stuff first. Wish I had.
1969, X-44, Export Model, Hugger Orange, Std. Int., BC Rear(soon to be a BT 12 bolt), PDB, GMPP 350HO, M22 Supercase, SSM Lift Bars, AirGap, Holley 750, FlowMaster American Thunder Exhaust, FlowTech Headers