You need to do a pressure/vacuum check of the fuel pump. Even a new pump could have problems, but I think I know where this situation is going. Start by giving the carb an independent fuel supply, like a 1-gallon can with a piece of copper tubing soldered into it, hung from the hood somewhere above the carb. Gravity flow will give you enough gas to idle, at least.
A good fuel pump will develop 3 to 5 pounds of pressure at the outlet line, and hold it for a few seconds after the engine is shut down. Then go to the inlet side, attach a vacuum gauge, and idle the engine. You should see 12 to 20 inches of vacuum, and this should also stay for a short time after shutdown. Check every connection from the pump to the tank, looking for a major loss of vacuum, or a hole in a line, etc. Don't forget the flex line from the pump to the frame. At the fuel tank connection, there should still be almost the vacuum reading you got at the pump, and it should remain after shutdown for a while. If everything checks out at the tank fitting, remove the tank and have it cleaned at a radiator shop, or replace it. I believe you've got a restriction inside the fuel tank, but check the whole system to be sure.
Jerry
