If'n ya take a gander at a modern body-on-frame vehicle, you'll see that seat belt mounts are to reinforced areas of the steel floor with larger grade 5 bolts. The steel reinforcement looks like a doubler plate so that the bolt does not pull through the sheet metal, either through a catastrophic event or from daily use.
Lost was a pic of a Pilot-House that I found on the internets where that restored buggy clearly centered a light pole and was sitting in a salvage...apparently, the shiny bench seat was covered in Armor-All, contrary to product warnings, and the driver made a sharp left turn going too fast and lost grip of the steering wheel, sliding over to the passenger side to witness his restoration work's demise. So I look at seat belt usage in a more positive light as a means to avoid an accident on a Sunday drive as well as to reduce injuries in an aggressive accident.