Carb Help

I have a slight problem i hope someone can help me with. I have an 81 GMC 1500 w/ a stock 305. The problem is with the Edelbrock 600 cfm #1406 carb sitting on top of that 305. I just did get the truck running the other day with a brand new set of NGK spark plugs and now it idles perfect but when i drive the truck it has real bad low end power and then as it warms up it starts acting like it is running too lean but it is blowing a lot of black smoke (it might be from it sitting up for the last 6 months with out being ran). I have tried and tried to adjust it but it aint workin. Any one have any ideas. oh yeah by the way the guys at edelbrock say that 600 cfm is too big for a 305 but every one else says it is just right. what is the truth?

It's a bit big for a stock engine, but not by much. A 650cfm is a good match for a 350ci. Get the old gas out of it and see if it still does that.