2002 GT Engine Sputtering Under Load ONLY, Vacuum advance problems?

Hey guys, I’ve been a regular at reading the threads here for a few years but have never made an account before. I think I have “hit the wall” with my project car. Here are my mods:
78mm BBK throttle body
SCT iTSX Bluetooth tuner

The motor in my car is rebuilt from Fraser Engines in Michigan. I did a motor swap and had my brother in law install the motor because I did not have any time to do so. The original motor in the car was stuck to the transmission so he had to pull them both at the same time. When he put my driveshaft back in the car he did not seat the C clips right away and tore apart the tail shaft housing and it almost came through the floor on me. My transmission was completely wrecked and I needed to put a new one in.

I found a 4r70w and a driveshaft from a reputable junkyard near me. the transmission came out of a Ford Explorer. I completed the swap but I’m having a really large problem.

In park or neutral, my car runs great. Any time I put a load on it it sputters when I hit the throttle.

I unplugged each coil pack and found none of them to be faulty, I have pulled each injector and found the same. I disconnected the MAF while running and that made the engine die. I disconnected the IAC and it died also. I made sure my PCV system is not clogged. I unplugged the EGR while running and it only made it worse. I finally hit the wall and I was talking to my friend the only thing I couldn’t rule out was my cats being clogged. I unplugged each O2 sensor and I thought it sounded a little better so I cut my upstream cats to try and see if the sputtering/stalling would go away and it didn’t help at all!

I know multiple brains are better than 2 so please let me know any test processes/where to look next. I am truly stumped. thank you in advance!
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