Consult the kit car manufacturer for seat belt anchorage points,but use common sense. These may well save yours or your passenger’s life so anchor to load bearing parts of the vehicle structure. If an anchorage point looks inadequate then it probably is (and the IVA man will tell you so). Test an anchorage point out. If it distorts using your own strength then you can pretty much be sure it will be no good.
Where should I anchor my seatbelts?
Where should I anchor my seatbelts?