Later Disco 300's had different seat bases to earlier. The earlier ones have a bolt in frame (mine has 1995) and the later had a steel frame that was part of the floor (if that makes sense). The later seats have a wider seat base. I know this as i brought a set to replace mine. they can be fitted but an adapter plate needs to be made up to widen your existing frame (if thats what you have), and if you get electric seats this gets a bit more complicated due to the moters being at the base, and therefore you have to raise the seats also?
hope that made sense, and helps?
Andy