Long story short, I just swapped in an 09 WRX motor into an 05 LGT. I left the air pump disconnected. The front oxygen sensor was different, so I retained the LGT one because the other wouldn't plug in. There were a few other small differences like the AC compressor and bracket, serpentine belt, the evap fuel line, and other irrelevant things.
I just went to prime it (unplugged injectors, key on, engine off) and the fans are kicking on high. The temperature gauge Bee-lined it to the H, yet the car has been sitting for about two weeks and I haven't even started it yet, so I know it's not actually overheating.
Now it's not even starting. Turns over strong, all lights are on, cranking sounds great (fuel, air, compression confirmed), but it won't start.
My thoughts? There's some difference in the wiring harness of the intake manifold, or maybe some related sensors, that's not allowing the car to start.