I don't know. 5,000 lbs sounds like a lot of force, but when you are trying to dissipate the force of your weight moving 40 mph or so to zero in a fraction of a second, you may get up there pretty quickly. If you're just talking about a lap belt, its probably not a problem. Adding a 2.5' lever (the seat back) as a prybar amplifies the force. One guy I was talking to recommended only the lap belt. He said the B pillar won't hold much anyway. However I didn't see a sliderule or pocket protector on his shirt.
