The first step is to strip out the interior.  You will need to grind out the rusted seat bolts.  The lino and plywood floor is glued to the underlying metal floor with the world's strongest glue!  You will be left with lots of holes in the metal floor where the bolts were.  I used small strips of glass cloth and epoxy to seal them. There was a bit of rust, but nothing too serious. I left the roof as it was - it was in almost pristine condition and already had insulation behind it.

I managed to sell the old seats on eBay to a woman who had an American Diner in France and who wanted genuine US seat.

You will need an angle grinder to remove old bolts and rivets