That looks like L track. It's engineered to hold wheelchairs down in a collision -- that means it's supposed to hold tight to the floor even when confronted with "a heck of a lot" of force. If you could easily pry it out, that would mean they'd built it seriously wrong.
It seems a little strange that you wouldn't be able to get to any of the nuts on the bottom... somehow somebody reached there to install 'em. Maybe they're covered in foam, undercoating, or other later-added accessories make them hard to reach now. One trick I've used with the impact wrench to avoid the need to hold a backup wrench on the other side is to pry the pieces apart while pulsing the impact wrench. The prying gives just a little pressure between the nut on the back side and whatever it's up against, and each time the impact wrench is pulsed it strikes against a nut that is stationary, so might get just a few degrees of loosening with each pulse. With the bolts so close together you might have to work several of them just a little bit each because the track won't bend much when it's securely bolted just 4" away. It really seems like it would be worthwhile to find a way to get to even some of the nuts from the bottom; each one that you can get will come out easier that way than any other way. Maybe you can modify cheap harbor freight or pawn shop wrenches to build a custom tool for the job.
Hacking that track apart at every 4 inches sounds like an awful task. What about drilling the heads off the bolts? Maybe rent a plasma cutter, air compressor, and air dryer and burn the heads off? (worked great for rivets in my bus!
) Or take it to a welding shop that has such equipment.