Seat belts are a good thing and will surely keep you in the car or “on the floor pan” so to speak. After reading about the dune buggy wreck above! Sounds like Michael Waltrip at Bristol in the Penzoil Busch car back in the early late 80’s.
If there are no provisions existing, you can weld plates to where you want to anchor the lap belts. I used eye bolts and the hooked style lap belt anchors. The “hooked” end has a spring loaded safety.