Was it running ok before you replaced the tank? If it was, its unlikely that the lift pump is the culprit.
Check all pipes and unions are all tight to prevent it drawing air, but equally important if the tank has been moved, make sure no rubber hoses are kinked restricting the fuel supply. I had this on a Ford Granada I changed the tank on.
The way the pipes ended up along of the tank when it was pushed up into place meant they sat at an angle and restricted the amount of fuel getting to the carb when the engine was under load. From the underside it looked ok, but it was only when the tank was lowered I could see the kink in the hard plastic.
Duncan