Back in the early seventies I had a 64 Grand Prix. Wide ratio Muncie four speed, 3.42 posi, 421 and two Carter AFBs. It ran great, it would stay up with the new Crazy KZs that just came out until it ran out of fuel that in the top of second gear.
My friends dad was a port authority mechanic and really knew his stuff. One day I was looking at the engine and he happend to walk by, he took one look at the engine and said; "that big Carter fuel pump needs fed. The gasoline flows to the back of the tank during take off and the car runs until it uses all the fuel in the line, his Saturday, bring the tank down here first thing in the morning.
Keep in mind, this is the most I ever heard this man say in my 25 years of living next to them. He was a big Polish man who did not like a lot of talk ad people who talk too much lol.
I was there, at 6 am. He yanks the big tank out of my hands like a gorella, put it in his stationary tubs and starts filling it with water, then says go home and eat stop back at noon and it will be done.
I come back at noon, he made a sump the tank it had two 3/8" ports. One was capped. It was a work of art. Everything he did was perfect!! I was smiles ear to ear.
After you install it and try it out, bring it down here and we will tune it up. Your running too much advance at idle and I can hear it spit.