Right. The later seats with the wider track do not sit on a removable frame, its welded as part of the body so you cant take it out unless you cut it out and can weld in to the older body (and have the measurements to get it in the right place).
Best you can do is make up a plate to fit the wider base seat and then bolt this to the narrower seat frame. Be warned though, this will result in the new seats you fit being about an inch higher - not ideal if you're 6'+ like me! Forever ducking to see traffic lights and you only effectively see out of the top couple of inches of the windscreen!
