[Kevin in NJ]

So a seat belt need to handle hundreds of pound of force with minimal forward movement. In the A it is kind of tight, that is you dont have to move far to hit something hard in front of you.

First rule of seatbelts. The mount must be as close to the shear of a bolt as possible. That is where the bolt is strongest. This means the belts need their mount point behind the seat a bit.

The seat belt path must be striaght through. I have seen several installs were they did the S turn around along the seat bottom. For my cabriolet I cut a hole in the metal curved part of the seat so the belt can go straight through.

You must be in something solid. Since the belt is behind the seat the force is going to be pulling mostly forward with some up pull. I have a 1/2 steel plate attached to a couple of cross members and under them. I have some wood sills and that is a bolt through the wood.

There are a lot unsafe belt installs using just all the wrong materials and attachment options. By unsafe they are not likely to be much for even just holding you in the car let alone keeping you in place.

The belts in the A are going to be a compromise in the end.

Remember that 25 MPH would be the same as taking your car up by the rear bumper with a crane and putting the front bumper at the level of the peak of a 2 story house. Then letting go. You need the parts you are attaching to to hold for that hit. I use 25 cause that is the speeds where you are most likely going to hit. Much above that it is kind of a crap shoot. Based on past accident pictures, the A does survive well on frontal impact. You are going to be injured, but more likely to survive.