is pressure suppose to build up in gas tank?

I changed Fuel pump it came with harness so I switched that out also,but I think the two black wires were not in the same order as the old harness,solid black and black with white stripe. ran fine then gas startd blowing out of seam and now it won't start

Need to know---Did you remove the truck bed or the gas tank to replace the pump module?

gas tank

IF pump is working.... Note that fuel lines and 'componet' connections are easily damaged while installing the tank... Suggest checking leak area very closely to pinpoint the actual leak source. May just appear to be leaking at seam... If you ever need to install another in tank pump in a pickup with a standard bed, remove bed to install. .. *Much* easier!

so why do gas shoot out of tank when fuel pump pulls gas to engine

Pump PUMPS gas TO engine under 'pressure'.. I can't see the vehicle, just suggesting to make sure exactly where leak is before attempting repair... Good luck!