After taking a few weeks off from the underbody project, I finally made some kind of progress. I finished trimming out the first piece that hangs under the cabin, drilled and mounted it! I wound up having to trim out a decent little hole under the downpipe since the panel had to curl upwards a touch to mate up to the beatrush undertray but I'm not too worried about it. I hate that I still had to cut a hole but I'm not going to modify the downpipe which is a plug-n-play part right now. The hole is far enough forward that it hopefully won't be an issue.
The panel is sagging just a touch in the middle so I'm going to have to add some reinforcements to stiffen it up, but otherwise fits great!