Help: Engine swap

Just switched a 1998 cheyenne 4.3 vortec to a 2002 4.3 vortec. Low fuel pressure. Can turn key on and off several times to prime, starts and runs for a moment. Seems like fuel pump to me but it was fine before swap. Amazing how that can be. Any ideas before i replace the pump, is there maybe some difference between the 1998 and 2002 like fuel pressure. Also i was told if i unbolt one shock and jack it up and remove one strap off tank it will drop down enough to replace pump. Is that true? Any help or ideas? Thanks

I'm having a fuel issue too but as for getting access to the pump ,the easiest route to go is remove the bed. atleast enough to replace it.