2000 Vortec 350 Hard start and stall

[GMC Burbalade]

Vehicle is 2000 Cadillac Escalade. I've been troubleshooting this for weeks and while I have made progress, I still have not gotten this to run.

Bought the truck unable to start, seemingly getting no fuel.
PO had replaced battery, fuel pump, fuel filter, plugs, wires, and ignition switch (wires were cut and spliced)

I replaced the fuel pump relay, reseated plug wires, and swapped out the spider injector for a known good unit. Fuel pressure is about 63 psi to prime and drops to about 55 psi when it starts.
Truck is now starts, but is suffering from a bit of a hard start, and dies 1-2 seconds after starting. Security system seems to not be functioning correctly. Sometimes the light flashes, but usually goes off after the bulb test.

Key relearn caused the security light to flash again, but was back to normal the next day. Cheapo ebay ELM327 OBDII scanner will not communicate with the PCM, but works fine with my Ford work truck.
Swapping ignition switch out for a spare didn't help.

I don't want to fire the parts cannon. PO already did that and it obviously didn't work. Bad PCM or does this sound like something else?

 

[Unkel Dale]

Truck ever get into deep water flooded? start checking wire harness continuity?

 

[GMC Burbalade]

Ended up being the security system. I was doing the Passkey II relearn when I should have been doing the Passlock relearn. I've got it going now and have been DDing it for a few weeks.