Ok - here's some thoughts from a safety person. AKA 10 reasons why I think you'll kill someone with this because of your lack of even the most basic design considerations:
1. Your seat belts will fray because you didn’t think far enough ahead to put a grommet in the bed. Fraying will be the most likely way that a seat belt would fail. I’d also love to see your actual mounting for the seat belts to see how far out of the mounting guidelines you are.
2. Single shear mounting of the seat bracket. If you’re going to do something stupid, do it in double shear
3. Judging from the picture, the welds on the brackets look like they’re sitting on top of the plate with limited penetration. I’m also guessing that the welds are bad because you didn’t even take the time to finish grind the bracket itself, so why take the time to weld well
4. In a roll over, you would decapitate your passengers. Best case failure is that they’re only severely injured. Those are not odds I would like to play with.
5. The bed has no structural integrity in a side impact, unlike the cab
6. It’s nothing like the seat in a supercab, which has to go through laboratory testing and meet minimum requirements from consensus organizations like ANSI. Rear supercab seats also have a shoulder belt IIRC. This is what would make the biggest difference in a collision when you’re sitting in a real engineered seat. A lap belt is only marginally better than having no seat belt at all.
7. Knots in the seatbelts are not good
8. No rear mounting points for the seat. Again - if you’re going to do something stupid, at least over-build it to the point where obvious failure points become less likely.
9. The bed sheet metal is much worse than the cab sheet metal for things like mounting – it’s not built for it, and you’ve decreased the integrity of the bed just a little bit more. I wouldn’t be surprised in the least if the mounting points end up failing because the bed, not because the brackets themselves fail
10. Frame flex offroad will do some very interesting things to the way that the seats are mounted. I’ve seen ranger beds to some really crazy things, even doing some mild offroading.