Sorry, still groggy from the long weekend! It's the front seats, the part of the floor with the mounts on it. Such good news that it's a match!
Should I just use a sawzall to cut the sheet metal out?
Have you checked my build thread recently? I just built that EXACT panel in the last few days... The only really tricky part is getting the flange bent across the top where all those carriage bolts go into it from the rear bedfloor.
The hardest part will be getting the vertical panel separated from the B-pillar and outer rocker sheetmetal cleanly. It's pretty well integrated with those parts so the challenge will be how to remove it without making a huge mess of the other parts. It might also be challenging to get the new panel into that location from underneath with the frame and bodymounts in the way.
-G