Have done it. You must order the covering kit special from Rovers North. Julian/Exmoor has the patterns for every Land Rover seat ever made. Talk to Arthur up there at RN. They do not have new back foams so if you can save those, you are golden. However, i will say that having been through the ordeal that it is actually far less hassle to simply order new seats. There is a real possibility of damaging the material as you are installing it, and then you will have to wait possibly weeks for a replacement part to come from the UK. All you are going to be saving from the old seats is the frame, and that, once you get down to it, is of little value especially if it is rusty or needs to be cleaned up or refinished. The tray backs are actually not as nice as the next-gen seats.
While you are at it, order them with the seat heaters installed.
1966 109 5 door wagon 300Tdi "spermaceti fueled"
1994 RRC LeWiB "ruining the air behind me"
All my troubles are Rover