Swap Startup: Stumbling

[cebra]

I am done with my swap and have asked a lot of questions on here that have helped me along the way so I appreciate the help. This is hopefully my last issue to resolve. The motor stumbles when I press the accelerator and acts like it doesn't know what to do. We hooked a scan gun to it and 1. it says I have a 5.3L 4x4 ECU and 2. it shows a low MAF reading. I swapped out the MAF and that didn't change anything. It is in the stock Silverado location by the airbox as well. I also called BP Automotive who flashed my computer and they said that they flashed it for a 4.8L per the detail I sent them. Any thoughts/things to test?

Side note, I can't actually drive the vehicle until my tires come in as the wheel are too small to clear my suspension.