Yes, seat belt bolts are 7/16ths UNF, but rear tabs are not seat belt mounts. They are smaller (3/8ths UNF?) and actually used to mount old style seat runners, but are often used as lap strap mounts with some adaptation.
As your car is an SV you may find it easier to use the same mounts as the lap belts with appropriate long bolts (it's a tight fit on an S3 -assume it's easier on an SV).