Putting the pass o2 in the downpipe is fine. Done it for many many years.
The o2 will read the same there or right before the turbo.
The glitch comes in. If both sensors are in the same area. Like driver side stock. Pass side in the crossover before the pass cylinders are included.

I will say its that or a bad injector.
Had this issue years back. Truck ran fine then started acting up bad.
Swap the stock injectors back in. Swapped the tune. Truck ran fine no stumbling at the rpms like before. Nor stumbling tell the injectors ran out.
Swapped the other injectors back in. Swapped the tune.
Stumble back.
Drivers side had a bad injector.
Wide band read fine. The computer seen a lean condition on drivers side so the PCM dumped more fuel in that bank.
The other 3 injectors picked up the slack. But one cylinder wasn't getting much. Luckily I didn't burn it up.

 

I bought them used from a member on ls1tech. He said they only had 1500 miles on them. Ill try fixing the exhaust leak and moving the o2 and then if that doesn't do it I'm going to order brand new injectors

 

Does it have a MAF?

 

Yes, the maf is 8" below the tb

 

Is it rpm, load, or boost that you know when it's gonna happen.

 

When I'm accelerating it does it bad. Worse when I build boost. At idle it'll act like its trying to shut off and then it'll start acting normal if I shut it off and cut it back on. Also as I said, under boost I get a thin bit of black smoke.

 

 

Post pictures of the kit...

 

I'm getting justin to help me with the starter tune then I'm going to dyno tune it

 

It won't let me..I've tried a few times. I don't have a photobucket or anything.