Nice looking truck, and the opening doors allow nice view of interior. There's been a discussion on RPF about 'Big Trouble in Little China' truck. which has those, and you can get them from http://kitformservices.com/photoetch.html PE85(it's down near the bottom)
the problem with the three-point belt is you need to connect to the roof and the floor, which is hard with the way they are assembled with the chassis going into the body during final stages of construction. sure they could just do the lap belt, but that isn't really correct for anything later than 60s.