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If Im not mistaking there is a low pressure in the tank and a high pressure in the frame rail under the driver door. The pressure sounds right but maybe getting ready to go out if the pressure is going up and down under a load.
It may not be a fuel pump problem at all! Have you checked the fuel pump relay's, or just even replaced them? Is it the low pressure pump or the high pressure pump under the drivers seat on the frame rail. 40 P.S.I. is normal pressure. Have you changed the fuel filter when you changed the pump. A faulty relay could being causing the pump to fail! Are the supply or return lines kinked? Is the vent line for the tank in good shape. The plug may have been the problem from day one with a poor connection causing overheat condition on the pump. You need the low pressure pump to prime the system, it is that hummmmmming sound you here just before you turn the key to start the motor.
If it ends up being the pump again there is a reason, and I would try to find it, cause replacing that pump is a pain in the *** and expensive. Especially if you got a full tank of gas!!!