Engine Stalling after swap

[couchonwheels]

Hi,
Swapped an engine on 02 Buick LeSabre and it's stalling after starting. Before I swapped it, I put new AC Delco plugs and wires along with intake manifold gasket and plenum. Also did a smoke test with the holes plugged when the engine was out to make sure there are no leaks. Connected everything and engine cranks fine but I saw some misfires on the cylinders.
In the time that it stays running probably a couple seconds:
Cyl 1,3,4,6 all have less than 8 misfires
Cyl 2 and 5 have Abt 20 and 55 misfires consistently
I swapped the coils around and the misfires didn't change much. So I thought it was an ICM and swapped that with no change.
Reviewing more OBDII data, the upstream O2 sensor is stuck at 447mv along with short term fuel trim is at 0. Googled the voltage and 447 is close to the ideal air-fuel mixture so I'm inferring short term fuel trim is 0 due to the O2 going dead.
The engine idles rough when I give a bit of gas but as soon as I let go, dies.
I have another O2 sensor coming so I'll do more tests then.
I also cleaned the throttle body.
I'm not sure if I have to clear any previous setting but I'll look into those as well.
Any help will be appreciated. Thanks

 

[couchonwheels]

Wanted to circle back through this. Did a fuel pressure test and the pressures were good. Through through more sensor data, noticed that MAF sensor values seemed high right before stalling. More like 14 g/s. Unplugged the MAF sensor and the engine came to life. Will be doing more checkouts to get it running good. Idling high right now but I assume it's because of default maf values. There are no misfires either

 

[02LTD2.0]

MAF should be about 4 g/s. Did you try to clean it yet?