Well brother I feel your pain but mine is a little different. My motor is similar to your's but my motor is a 350 with world product sportsman 2's, edelbrock rpm performer with 9.1 compression and cam is a hydraullic roller wich is 222/230 @ 50 510/530 lift and I get around 14 inches of vacuum at idle. I got a brand new holley 670 aluminum street avenger and I had all kinds of problems with just bolting it on. Engine would just fall flat on its face and go too lean and cause the engine to quit and drop cylinders cause lack of fuel. Thought something else was wrong and spent over two weeks of long weekends working and checking this and that to find out it was the carb.
Well got a new one to replace the faulty new one and it came out ok but the carburetor is way to lean. My plugs are bone white and I have already went from the stock 65 main,68 primary to 69 main and 74 secondary and my plugs are still white with little to no change and I have a off idle lean stumble and it back fires through the carb if you rev it fast. I bought some bigger squirters and will have to go from there. I have my timing at 16 degrees at idle and is 32 all in at 3000 rpm. I hope you can find out whats wrong with it. I checked my transfer slots as well and you won't be able to have the rear ones at .020 or anything close to it. I reclosed my secondary's and opened them up several turns but without the transfer slot showing. I readjusted my front to show about .040 of the transfer slot and it idle's excellent at 800/900 rpm and the idle mixture's screws are about 1 1/8 turn. Hope that might help you out as well. Also might want to try a different pump cam assortment and squirter's if needed and also go down in jet sizes if needed. I would die for tan plugs now. Still trying to get mine a little richer.