Good call on checking the bench seat. I specifically order that one for my new truck and even buying new seat to retrofit the truck can be cheaper than changing the truck.
Coming to safety, we did lot of stupid things when we were young that we no longer do when we get wiser.
I lost several of my young days friends who were not that lucky.
My first ClasC, back in 1980's did not have any belts in the rear.
That was legal back than, but even if that could be legal today, I would add belts anyway.
Lot of buses have no seat belts, what might be another story.
Good luck on your project.