Replacement floorpan sections are fairly inexpensive from usual sources. The DS Floorpan is not flat. There is a depression for the footwell and mounts are needed for the gas pedal. The panel has a couple of ribs in it to add strength. It is fairly simple to replace. A rotisserie is nice, or if drivetrain is out front and back, the car is down to around 400 lbs. and it can be carefully rolled on its side, so it can be welded on bottom as well. I'd be very careful removing the transmission tunnel. This is a major structural member and careful bracing is needed before rewelding. Otherwise, doors will not close and car will not track straight. I'd consider tunnel patching vs. wholesale removal. Sounds like DPO screwed up this car. Your job is not to screw it up further. Safety first. I'll defer to others but be careful before cutting. A couple of good You Tube Videos out there.