So this is finally resolved. It was something stupid of course, but something that wasn't thought to check. I planned on swapping the engine because that was the only thing I hadn't swapped out. I removed the exhaust and decided to try it just for the hell of it. It fired up and revved right to 6500rpms. So I immediately knew it was something with the exhaust. I removed the muffler and tried it, it still did it. I then removed the flex pipe, and it still did it, then I removed the small pipe off the header. That's where I found a small metal object, that looks like the cap to a metal chain link fence wedged in the pipe against the o2 sensor. I don't know where it came from, how it got there, or why it was in there. I do know it wasn't there when I assembled and installed the exhaust, or at least not in view. So it's aggravating that it's taken 2 years to find this, and to not know where this piece came from, but I'm excited to finally be able to finish this piece of the project and drive it!
Nice! I just read through this entire thread hoping you'd find what was up. Glad to hear it's fixed - gutted it took so long to find.