My 1985 (2 bodies) didn't have them from new and I think that Scouse is correct that Land Rover got away from having them for some years possibly well after your 1987 model.
What I would be concerned about would be bolting them to the floor. Not only does your 1987 probably have the aluminium floor, where the anchorage can rip right through the floor panel (the rear seatbelt anchorages are strapped to the chassis via a bolt under the floor), the seat itself can collapse. For the same reason, I would also be concerned about strapping them to the seatback, there was a family who were members of the Range Rover Club of NSW back in about 1988 who had 2 children die in a serious accident where the body deformed and the seat collapsed rendering the childrens seat belts useless.
I would be attending a seatbelt fitting station and having one of the restraint bars that are used for seat belts in the rear of vans fitted. (Or at least having them supply the bar and fixings then fitting it yourself.)
Diana

