Hi I have had both front and rear static lap and shoulder belts fitted into my MkIII GT and the company who did it made a fantastic job even if a little pricey the contact ifomation for this company follows
Quickfit SBS
Inertia House
Stanmore
HA7 1EP
Tel: 020-8206-0101
Quickfit SBS offer bespoke belts, harnesses, restraints to the industry or the end user. Our products range from the mobility market with harnesses and wheelchair restraints, chair lifts restraints through the automobile industry for seatbelts, MOT failures, damaged belts for the normal vehicle. Luxuary cars, belt convertions and classic cars, period belts to enhance the quality of the vehicle. Fitting and design services available.
Hope this is of some help
Martin