stumbling, bumbling sbc???

Hello everyone, I have kind of an interesting problem. This pull truck that I am considering the 400 sbc for has a stock mid 70's 350 in it right know. This is the motor I am going to finish out season with. It came in this truck when bought last year and I have no idea as to its history other than it has been apart before. It had a stock Q jet carb with the choke stove set up for the choke plate. Anyway, when pulled this truck and the four barrell was opened, it spitted and sputtered all the way down the track. If you let off slightly it would go away. I have decided to try a 1407 750cfm edelbrock just to see if motor was not getting enough gas. this carb I would think would be plenty big to feed this stock engine, but when was put on, the problem seems to have intensified. I took the sock off of the sender in gas tank and replaced and checked any bad fuel lines from front to back and all is good. I also checked to see what fuel pump was doing and is putting out 7psi according to gauge. the vaccuum advance is also working, but do not know what it is timed at since this is not an engine I built. I am looking for ideas as to what to check or try. I realize this edelbrock may be too big, I cannot pull this truck until I get proper amount of fuel or works properly.

oops double post..

un-plug the vaccum advance and set the timing a little high and see if it still does it. i dont trust vaccume advance. make sure you plug the vaccum hose back off. if it dosent go get you a good mechanical advance distributor, or just leave the vaccum un hooked and time high. i could almost garauntee that its in the distibutor.

Take the air cleaner off, and have someone else push the gas pedal to the floor while you check to see if the carb is going full throttle. I'll bet it is not.