not sure if you've seen this
http://www.camsmanual.com.au/pdf/genera ... 20Q207.pdf
3. MOUNTINGS
The safety harness or seat belt must be securely mounted on at least two points (Type D), three points (Types B and C) or 4 points (Type A). On cars derived from series production vehicles such mountings shall be positioned to provide compliance with the prescriptions shown in Drawings I-1 and I -2. If the two shoulder straps join prior to a common mounting point then that junction shall be at least 150mm behind the wearer?s neck. In all cases of mounting the following must be observed:
(a) On series production cars, some or all of the original seat belt mounting points may be satisfactory. Where the original seat belts are affixed to the seat, such mounting points may be used only:
(i) where the original unmodified mounting points on the seat are retained; and
(ii) the vehicle is being used for non-FIA International events.
Where the original mounting points are not used, additional floor mounting points must be reinforced with a 3mm steel plate of at least 75mm x 50mm on the underside of the body.
(b) Full harness (Type A and B) rear mounting points must be to a substantial part of the vehicle?s structure, reinforced as for floor mounts above, or to the safety cage.
(c) Under no circumstances may a safety harness mounting bolt be used to affix a safety cage to the bodyshell.
I have a cross brace for mounting the harnesses on my car.
I wouldnt be bolting the harnesses to anything except an original seatbelt points without reinforcement, I dont think that fixing to the parcel shelf will cut it either.
ohh and for those with more modern cars dont use those baby capsule mounts, pretty sure they aren't rated for a full size adult.