My first 'car' was a 76 Jeep J10 truck, I was 15. I was only supposed to use it to drive down local private roads that led to a couple of local ranches I worked on after school, but I did take it out for a couple of joy rides. Three on the tree, burlap bench seat, lap belts, and a radio.Thats a very good point to make! Growing up in an era where airbags are standard, kinda makes me fear this car.
That said, I echo everything nihkon said. Either OEM/reclinable seats and OEM belts and no roll bar, or full bucket (FIA spec), five point (FIA spec), roll bar or cage (to meet the minimum requirements for any sanctioned racing series) and off road use only with a helmet. If you go with aftermarket seats for a street car, they need to be reclinable, and made by an OEM supplier. I'm partial to Recaro.