I have a 98 k1500 5.7l vortec. New parts include TPS, Camshaft Position Sensor, IAC, MAF, Cap/Rotor, Fuel Filter and a new Fuel Pump. Issue i have is it will stumble upon acceleration. It will occasionally backfire through the intake.

Key on Engine off pressure is 50 PSI.
At idle pressure is 51-53 Psi.
WOT it jumps to 58-60 Psi, but does not stay.
If i gradually rev up, PSI goes up then gradually back down to a constant 53 psi.

Fuel pressure Regulator?

 

53 PSI is way too low. Should be closer to 60 PSI. Bad fuel pump possible.

 

Using a pair of hose pinching pliers, clamp off the return line where the line passes from the engine to the frame. Run your fuel pressure tests again - clamping the return line deadheads the pump and prevents/restricts fuel from returning to the tank, so expect it to be at least 70 PSI. If the fuel pressure remains above spec for KOEO and KOER tests, the regulator is likely at fault.

That said, these injector assemblies are known for broken lines, and the regulator is mounted on the injector assembly, so unless the injectors have already been replaced, I’d recommend just doing everything while you’re in there. It’s more expensive but at least you won’t risk removing the intake twice.

Additionally, a backfire through the intake indicates timing problems - the pressure of combustion couldn’t get into the intake unless the valve is burned or open at the wrong time. If it’s intermittent, this suggests a spark control issue, but I suggest you first double check that the mechanical timing of the distributor is correct.