Nice car I like the mk3's, me dad had a 1980 xr3 in silver, with the alloys you speak of.
The mounting points for the seat belts are all there, lift up the seat base and you will see them, then the rear parcel shelf side supports will have a section where the belt passes through to its mounting point under neath the side support, the pivot point for the belt is on the rear pillar bit, may need to either remove a bung or even drill out plastic, should be marked on reverse on trim part if needing drilling, but structurally all mounts are there, my dads had rear belts as std, but was german spec but rhd for forces, so possibly uk ones didn't have as std ?, german ones also didn't have the correct lamp holders for rear fogs either, but a simple join with wire is all that was needed so I know there are differences.
tabetha