I can see that there is a previous 'thread' on static rear seat belts, but does anyone know if inertia reel type can be fitted, to a 1984 GTL with the normal fitting points?
It should be possible to fit inertia reel belts on the original mountings. You can buy universal ones which can be fitted at any orientation (ordinary inertia reel mechanisms only go one way up). I have a set in my MGB from Moss Europe.
Positioning is a little more tricky. Probably the best place is where the static belt upper mounting was originally fitted. That'll avoid eating into boot space or mounting to the wheelarch which will probably be a bit rusty.