Chevy 5.7 needs starting fuild at first/then it will run all day . Good pump, relays and pressure ALL CHECK OUT.Where do i go next

well learn how to tests spark, all spark plugs not just 1.
if sparks is bad, and the PCm detects it bad the PCM cuts fuel
so guessing fuel first is dead wrong, and you took a bath in fuel for nothing, wow that is brave.
cranks but will not start and run.
4.3 is old push rod engine, no cams in head,
fuses good and now scan the PCM after cranking for 5 seconds do not turn off key let the tool see there errors now on any dead engine.
sorry you need have scan tool if engine actual is good.

the rules are. in order

1. engine good, egr not stuck open or CATS melted.
2. spark tests good on all 6 cylinders. google how and learn
3. if spare is good we try test fuel next, instant start in a can spray.
4. if that works we check fuel pressure next. (and last see why?

#1 engine passes a compression test, (a TOOL)

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Replace the fuel pump relay.

Maybe a problem in the fuel pump circuit. Check fuel pressure with engine off, and key turned to on, not start. If no pressure, diagnose the pump circuit at the fuel pump relay. One terminal for the relay would be the power feed for the pump. The engine computer signals the relay on for 2 seconds with key on to prime the system, also signals during engine cranking.

Check the fuse and fuel relay. Relay is located in electrical center under the hood on the driver side. The relay and fuse legend will be on bottom side of the protective cover. You can test it by exchanging it with another relay that you know works, (horn relay, etc.).

With the key OFF, listen carefully for a slight whirring / buzzing sound for 5+ seconds or so when you turn the key to on / run. This sound is the fuel pump running. If you hear it run, then it is most likely pumping fuel - but is blocked somewhere (crushed line?). You could disconnect the fuel line to verify fuel delivery while the pump is running if you need to. If you don't hear it; check for 12V + and GROUND at the pump. Remember, the pump only runs for a few seconds when the key is in the run position with the engine NOT running, so power will be there for a few seconds only. Check for loose / poor (+) and ground lead connections as they can cause this problem too.

Good luck!

Power to the pump is provided by the relay for the first 3 or 5 seconds to build pressure back up to start. Once the 5 seconds are past, the relay is grounded by the computer IF it sees oil pressure or crank signals. This helps prevent a fire if the car is in a wreck.
If it still won't start with the pump relay bypassed, you would check for spark and see if the computer is getting a crank signal.

Testing is the way to a fix . Fuel pump pressure testing , electrical testing . Viewing engine sensor data on a scan tool . Instead of guessing as to the problem learn to test or take it to a qualified repair shop . Changing ECU without doing it correctly can cause problems that were not there before . PCM - engine computers need to be programmed , dealer only.
GM Fuel Pump Relay Testing
Testing Electric Fuel Pump Problems with Voltage Drop Test
048 Engine Hard to Start or No Start Scan Data for Starting Problems

It sounds like the fuel pressure check valve on the fuel pump has gone bad and your losing fuel pressure when engine is turned off. Try priming the fuel pressure twice, that's when you turn the ignition keys to the run position with out starting the engine for 10 seconds and then turning the key's to the off position and on again for 5 seconds. After the second prim the engine should start, to check this you can lone out a fuel pressure gauge and check the fuel pressure at the shredder valve. You should have 62 to 65 PSI during prim and once the engine is running, it should drop by 5 psi. As for your the accuracy on reading the right amount of fuel in your gas tank, the sensor is apart of the fuel pump sending unit and the float may have gotten bent and nothing you can do to fix it unless you take the fuel pump out again and replace the sending unit. Good luck and keep me posted.

Fuel pump is overheating (means it is failing) replace it.

Have you checked that you have power to the coil on cranking?You may also want to check wether you have a good strong spark at the spark plug.It could be an ignition module breaking down.If you have no power to the coil you may need to change the ignition relay.Its close to the battery.