5.7 TBI first start loud popping

[Parts]

Hi everyone. I have a 92 5.7 TBI from a G20 van that used to have a 7747 swapped into an S10. It's a known good motor I pulled a few years ago. I re-pinned the factory harness for a 7427 computer which I am using now. I am using the stock sensors that came with the motor except I got the correct knock sensor and O2 sensor for the 7427 computer. While cranking the motor I am getting a very loud popping sound. More like a m80 going off in my garage. I have open header on the driver side and enough exhaust on the passenger side to put the 02 sensor in (with no muffler) so that's why it's so loud, but it's a real freaky noise and I don't want to do something stupid and blow up this motor. Timing is set to 0° and distributor pointing at spark plug 1. I primed the oil pump before cranking and have oil up through the rockers. Fuel pump kicks on after a moment of cranking to build oil pressure. I had an issue getting the fuel pump to come on initially (bad relay) and ended up using the fuel prime connector to build fuel pressure and get my fuel pressure regulator set correctly @ 10 PSI. I'm not sure if these motors can flood? But I did dump a lot of fuel in getting that regulator dialed in.

EDIT: Turns out autozone gave me the wrong O2 sensor (the 1 wire version instead of the 3 wire version on the 7427 wiring diagram) so I'm assuming that's my culprit? Is there anything else I am not thinking about that could cause this motor to blow up?

[dave w]

Often a distributor misaligned will cause a backfire, which makes a very loud pop noise.

[In-Tech]

Hiya,
Advance the timing. It is virtually impossible to blow up an engine in neutral without an over rev...well unless it was doomed already :)

[MO LS Noobie]

I agree on distributer misaligned or maybe firing order. Basics first.

[TriumphR3]

10 psi seems low for your pressure regulator

[MO LS Noobie]

A little help on the popping noise. Is it a single pop when you allow the key to return from start to the run position, or does is it consistent with engine rotation? Meaning does it seem to happen once every engine revolution or every other revolution? Usually backfire through the intake is lean and out the exhaust is rich. Does the engine even try to start and/or run?