fuel problem

[PMS]

I haven't been here in a long, long time. Finally got my 72 long bed together and running but I am having fuel problems. I put a fuel tank from a newer suburban behind the rear axle. Replaced the 1/4 return line on the sender with a 1/2" line for a pick tube for the electric pump. I removed the in tank pump and used the stock 3/8" line as a return line from the regulator mounted in the engine compartment. The truck has been lowered 5/6 so the pump could not be mounted below the tank. The pump is mounted flush with the top of the tank. The inlet to the pump is about 20" in length total. 1/2" lines run to the regulator. When you first start the truck it will maintain 7psi but after driving for about 10-15 minutes the pressure drops to 3psi. I have checked the pressure with a different gauge and it is not the gauge. The electric pump is a carter vane style pump. Do I need a bigger / higher psi pump or will I have to have a custom tank built with a sump system? Have any on you had this problem?