I am having problems getting my ls swap engine running. It is a 5.3 with a ls1 intake. It has a BP Automotive (Awesome!) harness tied into American Autowire harness.
The fuel pump primes in run position. And the the engine starts. As soon as I let off the key it stops. VATS has been tuned out. As soon as it goes back to run I can still hear the fuel pump. Fuel pump is the speedway kit with msd pump and C5 regulator. It farts and stops. Sounds awesome then nothing. Where do I start?

 

Sounds like your 12v keyed hot in crank and run is hot in crank only. Check that it's getting power in both.

 

I'm not really familiar with either of those harnesses, but I would check to see that the coils are getting power when the key is at "run", maybe they are only getting power during cranking.

 

I'm going to assume BP Automotive either mods a factory harness or builds a custom one with factory wire colors. All Pink wires in engine harness should be hot in both crank and run. You said you hear the fuel pump run after the engine dies when in the run position. Does it run for 2 seconds and stop or continuous?