Panel seats are way different than Suburban seats, but anyway:
I did put shoulder and lap belts in a TF suburban years ago. You have to find places under the floor near structure and use fairly large, think plates supported by structure. The frame would be the ultimate structure.
The shoulder tie point should be about eye high and you will have to drill a hole in the sheet metal and fish a plate with a threaded hole in behind. I think I cut a slit at the door jamb or at the suburban side window. Be creative and stealthy.
Attaching belts to the seats of old trucks is a bad idea. The seats will rip loose from the thin floor upon impact. If you do what I did and the seat rips loose but the belt stays attached to the reinforces floor, you may get choked or have internal trauma. Also, many truck seat backs do not lock, so potential for injury there. I am familiar with mechanical design. Who knows if my effort will help or hurt. You can actually cause more injury or get trapped.
The jump seats on panels and suburban's are double fold. Do I have to say anything about that? Even the floor supports under the cab are basically sheet metal formed shapes.
The final analysis is that most kits sold by vendors are not designed properly as far as the anchors are concerned. Big washers won't hold 30 G's at 30MPH on a 170 lb person. 2.55 tons force. The higher speeds are gigantic G loads on your body. All points, including the seat hardware must withstand pull out, rip out, strip out. These vendors have no real data. They have probably tested their hardware but not as mounted to .xxx think floor or pillar of every vehicle covered by their product. If you install seatbelts, keep the anchors big, thick and supported. Be advised that the seat itself is dangerous in many ways. It all has to be strengthened. That is not mentioned by the sellers. They sell iffy backing plates for the belt tie points, not for the seats. They have disclaimers in their verbiage.
Something may be better than nothing or it may be nothing.
