I looked into this in some detail when I fitted mine. I expected to find a standard for the forces and directions on a seat belt mounting. I never found one, if anyone has found a standard with forces in do send the link. What I could find was about the bolt size and grade and the importance of chamfered edges on any doubler plates so they don't just cut through, all sensible enough. The only test I could find was the MOT which is a firm pull by the MOT tester. This got me thinking, why is it so hard to find specs for forces or strengh requirments?:
A lot of the vehicle will deform quite a lot in an accident, that's not necessarily a bad thing, it absorbs energy, what it must not do is trap or penetrate the occupants. So its OK to bend or crumple a bit. A very rigid fixing may not be better.
The belt goes around a human, it's purpose is to save the life of that human. Beyond a certain force the belt becomes the killer not the car body. The belt must do its best to keep a separation between the human going one way and the car body/dash coming the other. Its ok for the belt to stretch, mounts to bend, it may even help soften the decelleration. So it must hold, but it doesn't have to be elegant and beyond a certain force nothing more can be done.
My conclusion was, think it through, get the belts in the right place, follow the guidance on bolts and brackets, then give it a damn good pull. Taking the discussion above, I'm happy / confident in my bulkhead mounting becuase it has load spreaders, correct bolts etc, and it can take all my strength. A seat mounted belt would need to do the same, and that may not be easy because of the leverage from the seat back to the seat box.
In my case I was removing 2 fixed belts and a lap belt and fitting 3 intertia reels with shoulder belts and adding spreader plates under the fixings. I was happy that this was a safety upgrade and provided better protection.