There's a good thread on this subject, with several methods discussed. Personally, I put a diamond wheel on my angle grinder, and ran right down the middle of the track. You can then pry up each side, and grind off the bolt heads easily
Thanks for these ideas. Since I have just attachment points not rails, I'll try removing the top part of the device with my current cut of wheel first to see if it works before purchasing a diamond one. I'll post a picture soon that shows what they look like.
A nut splitter might work. I had the same system and rusted bolts issue. I was able to grind off some, sawzall a couple from below. One was obstructed by a toolbox from below so I ended up breaking out as much plywood as I could and using the sawzall between the anchor and sheetmetal floor, there was about 1/4" to work with.
The other day I worked on the removal of two attachment points. I made images of the process. The one accessable from below was easy with the angle grinder. The one from above with just the grinder, not so much. I tried to grind a line across, hoping to break the plate in two to remove and get to the brolt. I managed to do both at the same time, meaning Hitting the bolt head while cutting the plate.