Seat belts are an interesting topic. Some people hate them. Some people love them. Some people won't ride in anything without them. Some people use them because they are there. I suspect that anyone that has been in an accident where the belts left a mark but they walked away has a completely different attitude then someone that knows someone that was involved in a crash and the belts didn't save their friends life.

I have installed seat belts in a lot of vehicles, many that never had any belts in them. Some of those seat belt installations have been crash tested. Some were hard tested, others light tested. I can honestly tell you I never like to hear of any belt installation I've ever done that has been crash tested. I can tell you that in every case that was crash tested, every time, the occupants have come out better then they would have had there been no seat belts at all.

All that said, I have to add the disclaimer. I have no engineering degree. I have no CAD layout of your vehicle. I am showing you how I put seat belts in my truck. I accept no responsibility for any injury or death that may occur if you copy what I have done with my truck. Your vehicle, your responsibility, not mine.

Over the years I have looked at a lot of auto factory seat belt installations, and many in wrecked vehicles. The overwhelming conclusions I have come to are the factories use the best hardware they can, always grade 8 or better bolts, nuts and washers. The next thing they always do is tie the mounting location to as many other structures as the can within 12" of the bolts. If the anchor bolt moves, its moving a whole lot of stuff with it. A lot of modern stuff has the belt mounting points on reinforced areas of the seat frame itself, then the seats are mounted in the vehicle on reinforced flooring and brackets. If the belts are mounted on a sheet metal floor, the bolts pass through the floor and screw into an anchor bracket of at least 1/8" thick (many are 3/16" thick) that is at least a 2" square that has rolled edges that won't tear the metal. Those anchor brackets are also spot welded to the floor with 4 spot welds each.

If you are installing belts into a vehicle that has never had belts, I will suggest you buy new belts rather then use old belts, seat belts have a life expectancy, installing belts that are near the end of their life expectancy just doesn't make much sense. When you but the new belts, also buy the installation kit. It comes with the bolts, washers, and anchor bolt plates, and often pivot plates that come in pretty handy, that are often cheaper then you can make them for.

All of the factory retractable seat belts are designed to be mounted in the vehicle at a specific angle. The seat belt housing has a swinging pendulum inside of the housing that allows the belt to move in and out freely if it is centered, but locks the seat belt movement outward when the pendulum isn't swinging freely at its center. It the retractable belt housing is not positioned in the position it was designed for, the pendulum can not swing freely at its center and it will lock the outward movement of the belt. Generally the belt housing has a level line inscribed on two sides that if level (to the earth, not the car) allow free movement in and out of the belt. With a retractable belt, the retract base mounts on the floor, or into a specifically designed recess that maintains the level it was designed for near the door or the outer side wall of the vehicle. The belt pulls out and have a pivot point that is usually mounted fairly straight above the retract base. That pivot point is generally one bolt screwed into a structure with a lot of reinforcing. That pivot point allows the belt to change angles (to accommodate different seating positions) from there the belt has a slip movement male seat belt latch piece (this is the piece you pull across you to connect to the female end on the center side of the seat. Below the male belt latch the belt extends to either the seat frame, or a bracket on the floor. The belt can usually pivot on that bolt. The female part of the belt is usually pretty simple. The female latch assembly is attached to the belt. it extends to either the seat frame, or to the floor or transmission tunnel, or may be bolted to another bracket or extension piece. If there is a center seat belt, or if both seats are very close together, there may be another belt bolted to the same brackets. Most of the seat belt attaching points are pretty straight forward at the seat base or at the floor. The concept is that as the belt is mounted, you want the belt to pull straight against the brackets and bolts, rather that at a 45 degree or 90 degree angle. The biggest problem is the upper pivot point. That is what I'm going to spend my time on.
The goal is to mount that upper pivot point behind the shoulder of the seat occupant, with the seat at its farthest rearward position, and it should be located at or above the shoulder height of the tallest occupant. That goal can't always be met, but it should be the desired goal.
Pic 1, This is about what the male end of of the shoulder belt should look like when finished. This particular mounting is not correct for the pivot position in relation to the seat, The pivot needs to be much higher and more forward. but it gives you an idea what it should look like. This is the driver side, the passenger side should look just opposite. The next few pic will be of the passenger side of the truck.
Pic 2, This is a picture of the door post. This section is about 2 1/2" in diameter. I want to point out all the curves corners that make up the outer section. Every time you add a bend in sheet metal, you increase its strength. With the number of bends in this pillar, it is vert strong for its size. it would be very difficult for this section to bend top to bottom. This is the kind of piece you want for seat belt mounting, if you can mount the belt hardware without weakening the structure.
Pic 3, This is the inside section of the last picture. I want to point out that this is two pieces of overlapping steel. I also want to point out that the curve by the window has a pair of 90 degree bends about an inch apart. you can see I have several holes I have driller into this surface. The important thing here is to notice that the larger hole is where my upper belt pivot piece will be mounted.
Pic 4, This blurry pic shows 2 things. 1) this piece really is double thickness as you can see in the larger hole. 2 you can see that pair of 90 degree bends, What you can't see is that the outer piece of metal is also part of that corner window frame. If I drop a piece of bar stock into this rolled sheet metal section, that has a dilled hole for the belt's pivot bolt and a nut welded to the back side of that bar stock, and that bar stock is plug welded to the pillar, there is a lot of stuff that has to move for that bolt to move.