2000 Silverado 5.3L fuel pressure still low after new pump and more...

2000 Silverado 5.3L 2WD 1500 RCSB w/ ~165Kmi - all stock
Fuel pressure still just under spec after new pump and more...

I replaced the fuel pump (AC Delco OEM) and the new pump made only a slight difference and pressure is still out of spec. The new pump pushes about 2psi more pressure and draws 2amps less.

Spec per Haynes:
55-62psi key On / engine Off

Old pump:
52psi key On / engine Off
48psi at idle
56psi at idle when you pull the vacuum line from the regulator
Amp draw 8.5amps

New pump: 54psi key On / engine Off
Flutters between 48-50psi at idle
58psi at idle when you pull the vacuum line from the regulator
Amp draw 6.3amps

Fuel pressure regulator vacuum line is dry and no obvious signs of failure / fuel leakage and also see the readings above with the FPR vacuum line removed.

Measured the voltage drop for each the power and ground circuits and that checks out as well. Voltage drop is ~0.1v on the ground and ~0.5v on the power with the truck running. Battery only (i.e. not running) ~0.1v on the ground and ~0.4v on the power. Measurements taken from the battery post to the pump connector. Pump is also getting 14v while the engine is running (~14.6v at the battery).

Removed the fuel filter and blew out the supply line from the pump back to where the filter would be then switched sides and blew out from the supply line at the rail to where the filter would be and it did not make any difference. The fuel filter was changed about 2 months prior to the new pump in an effort to fix the low fuel pressure but did not make any difference.

Double and triple checked for any leaks anywhere from the engine back to the pump after the pump replacement and all looked good.

So all that and the fuel pressure is still lower than the spec. of 55-62psi key On / engine Off.

What should I try next?
What does your fuel pressure normally read key On / engine Off or at idle for a 5.3L of this vintage with return systems?

Thanks.