If you can, check whatt the harness manufacturuer says. From memory the best angles for a sedan based car, ie not single seater, is at around 10-30deg from horizontal down. Be very careful how you mount them and if in any doubt talk to an engineer, as this sort of thing is potentially deadly if you get it wrong.
The harness in my Zed is attached by looping the shoulder straps over the rear strut brace, which forms part of the roll cage. The last harness used the standard mounts bolted to the rear strut brace using collars through the tubing and high tensile bolts.
Where the lap part of the harness utilises the standard mounts, ie captured nuts, high tensile bolts were used, as well as the mount for the crutch strap.
Also check the age of the harness, for Circuit racing harnesses are now lifed at only 5 years, ie 5 years after the date of manufacture you throw them away, whether they have been used or not. I'm not sure whether this requirement is in effect for sprint events and so on.