Here is a quick overview of how I put seat belts into my cabriolet.
Here you can see the rumble center brace bolted in position and the front brace layed in position.
The sides are easy. The front seats are just bolts through the
wood. Ya, I know some people feel the wood will not hold. With a nice washer
the wood will hold up fine. If I have enough force to rip the bolt through
the wood then the seat belts are not going to be too helpful.
The center belt takes some structure. I was originally thinking something heavy running across the sills to re-enforce the wood. Then I realized that the seat belt mount will be behind the seat in a postion that means most of the force will pull the bar forward. I feel the wood is going to be strong enough to take a good pull. So I set up the bar to be against the back of the wood and a nice wide plate to spread the force. The back will mostly be pulled up and the long leaver are length will reduce the force. I only need to sort of hold the rear in position as the force will be up against the rear metal cross sill.
The bar is 1/2" steel that I had laying around and got with some other materail when I bought a lathe. The front is just a steel pad that will be wood screwed to the cross sill. The pad has a step to locate the top of the bar against the bottom of the tapered floor pan. I will drill and tap the 1/2" bar for a bolt. The pan will get an oversized hole at the tapped hole point.
The rear edge of the bar will be bolted through the rear edge
of the metal cross sill.
It will be painted black and is tucked up in and will be hard to see.
You have to be careful with the point that the tab come through the panel. The seat come
fairly far rearward.
The rumble seat belt sides are attached to the post the rumble
lid hinge bolts to. That is pretty thick metal and well attached with bolts
and wood screws.
The center brace is made of 1" thick walled square tubing. I cut 3 sides off at each end and then welded in a doubler at each end where the flat area i will screw to the bottom of the wood transistions to the square bar. The bar is held in shear and the point where it transitioned from the flat to the square needed extra strength. I also added a a little pad forward to help with any rotational loading caused by the "lever" I am using to attach the seat belt to in the center. The thin piece of metal will go through a rectangular hole I cut in my rumble pan. I will also be added some triangular braces for some more strength on my metal tab that will be the seat belt mount. The curved inner panel needed a notch cut into the front edge to clear the tab.
This is the final installed look of the rear seat belt tab.
This rumble brace is tucked up in the back edge. I needed to leave lots of room for putting the nuts in that hold the front edge of the curved inner panel. The bolts also have to fit in under the pan. I have the brace pretty far back as there is not much room behind the seat springs and that is the other reason why I have the second mount hole going forward.
I bought seat belts from:
They were nice enough to send me some short samples for color. I choose the plastic ended belts because I hope they will not scratch paint by accident like the metal ends would. I bought 76" for the front and 60" for the rumble. The standard seatbelt hardware bolts are a 7/16 SAE fine.
Please keep in mind seat belt installation involves risks. There are a lot of forces involved and what I am doing may or may not hold. You are using any of my ideas at your own risk and you understand I am not a professional fabricator or seat belt designer.
A little side note. This is actually the beat up trunk pan from my brothers 31 coupe. I kind of beat it into shape. I could have bought a repro that was missing detail or used an original that was sort of beat up, but did not look too bad where you would see it.