4-across the bench seat is a squeeze but we all have a seatbelt, i haven't checked the legality of that though, to be honest

My next thought is to fit a bench seat or a pair of rear- facing seats in the pick-up bed.
I'd securely fix them through to the chassis, have 3-pt belts and a roll-over bar, possibly even a bit of tarp for shelter if they're good

Any thoughts?