I was an early adopter of seat belts and car seats (for my now 46 year old son). I adapted a number of vehicles including an MGB-GT and numerous cargo vans.
Perhaps the most crucial part of mounting a seat belt was "fender washers." Invariably I would drill through the floor of the vehicle and use a small stack of fender washers to spread the load. I'd start with a 3/4" fender washeer then down to 1/2"...the size of the bolts I used to anchor the belts, then top it off with a lockwasher (nylock nuts didn't exist). Probably overkill, but I had no guidance, and I was an Ironworker, so, bigger is better.
The fender washers spread the load on a simple hole through the floor.
Seat belts and anchors are much more readily available and far more sophisticated these days
. No doubt, you could replace the factory lap belts with something that cinches up more tightly on your child seat. These adapters could be helpful if you need to tie into a frame member
Soon you'll need a "tether" to hold the top of a forward-facing seat and so on, so customize to your heart's content and make something that will really fit and keep your precious cargo secure.
PS, just make sure your adaptation is fastened securely so it can't become a missile in your vehicle.