I just looked up the vin Z 2002-2004 flex fuel 5.3L fuel spec.
key on engine off the pressure should be 56-58psi. engine on pressure must be above 48psi.
remove the vac line should go up to 56-58 psi.
If the pressure is not between 48-54psi engine running ,then I would next replace the regulator. the regulator is returning too much fuel.
not sure what fuel pump , for the E85 fuel. many numbers...
check that the regulator is for the vin Z engine when you purchase it.
replace the plugs with the ac delco irridium plugs.
do not attempt to gap plugs ..if the gap looks off they are defective so check plugs out before leaving the store.
this flex 5.3L is rare around here. fuel pumps and these regulators with the E85 is hard on these parts.