Fuel pressure drops rapidly

96 350 vortec has 63lbs of fuel pressure as soon as u turn the key on but drops to 58 rapidly and it runs horrible when u floor the gas pedal please help

when you try to floor it it spits and sputters and cuts out and when under a load runs bad what would cause my fuel pressure to do that and do u think thats my issue ive changed distributor, coil, plugs, wires, cap, rotor, crank sensor, temp sensor, and maf sensor I cant seem to figure it out please help me thanks