What Tony said.
Suggestion: You will probably find that the seat will not go as far forward as needed to reach the rear two bolts. At this point you may need to tap it forward the last few millimeters with a hammer. Be very careful here, the seat frame you will be tapping on is just steel tube, really. Use a block of wood to distribute the blows evenly, don't hit too hard, just enough to do the job. There are decades of crud preventing you from moving it forward all the way I'm sure. Clean before reassembly & spray a light cote of silicone lubricant on the runners so it moves smoother for you.
I also suggest you run a tap through the threads in the floor before reassembling, Clean the bolt threads, also use a little (you don't need much) anti-seize on the bolt threads, also.
Colin