Hi Gary
I have a 1979 900t, recently had the plates welded in the back for seat belts.
we had a plate welded on both wheel arches and a single plate welded in just above the floor on the vertical panel below the cupboard, we had to remove the fibre glass cover plate first.
when the plates went in we tried fitting lap belts and found we had to buy longer ones, we do not have shoulder straps just lap only.
hope this helps
Tim