Alright, here it is:
http://www.motorsport.org.nz/Pdf/MM34%2 ... ch%20A.pdf
Pretty much the whole of the following excerpt is relevant. Also, with a bit of searching, a lot of the answers to your questions can be found in the pdf I've linked to above. If you're serious about modifying your car to compete in motorsport events, it could be a good idea to have a read through the whole thing, there's a fair bit to take in but as you go through you'll find you can skip sections which aren't relevant to your car.
From Page 162, under the heading "Safety Harness Installation"
5) Anchorages: If installation on the vehicles existing anchorage points is impractical, new anchorage points may be created, in which case the following shall apply;
(a) For each new anchorage point created on the bodyshell, a steel reinforcement plate of at least 64 mm x 64 mm x 3 mm or equivalent in area with radiused corners, chamfered edges, that follows the panel surface with the attachment hole centrally located, shall be used, and
(b) Shoulder straps may be anchored directly to the roll protection by;
(i) Eyebolts in combination with inserts (Refer drawing 253-53), or
(ii) Eyebolts affixed to a flange consisting of a minimum 4 mm thick steel plate, wrapped a minimum of 30% around and welded to a transverse bar (the flange shall not be reliant on a ‘butt weld’ type fixing), or
(iii) Looping the straps around a transversal bar in combination with a guide to prevent sideways movement.
Additionally the straps may lean on an appropriately positioned transversal bar to enable the installation angles to be achieved (Refer drawing 4.3.3), and
(c) Where a HANS® system is worn the shoulder straps shall slope downwards (0 to 20 degrees – ideally 20 degrees) from the uppermost point of contact with the HANS® belt bearing surface to the anchorage point on the vehicle. (Refer drawing 4.3.3), and
(d) Each anchorage point shall be capable of withstanding a force of at least 12.5 kN, applied in the direction of the installed harness strap. Where two straps share the same anchorage point the force considered must be equal to the sum of the required loads, i.e.25 kN.
(e) Harnesses specifically manufactured for single seater applications shall comply fully with the manufacturers installation and anchorage specifications.