You have to cut a hole in the lower portion of the seat frame and run the seat belt anchor behind the seat.
I ran a 2" wide by 1/2" thick bar from the cross sill the rumble platform front end attaches to the back of the wood cross sill, from the bottom- in the picture i have it laying on top of the floor. I put the front of the bar against the back of the wood and welded a metal plate that screws into the wood cross sill. The idea was to translate most of the force forward into the long dimension of the wood. Keep in mind this is not a perfect solution, but it will certainly hold for a little bit in a real bad accident.
In the pictures you will see 3 rectangular holes I welded up on the seat frame. I made a single hole about mid way up that lower area. I then took brake line and slit it and welded over the edges to give the sheet metal nice round edges for the belt.
The side belts are also place rear ward. When installing seat belts you always put the action on the bolt in a shear as that is where you get the most strength.
I have also included my solution for a rumble seat belt middle mount point. That is heavy 1" square tubing. You have a very small place to put a mount point up through the rumble riser as the seat frame comes back pretty far. Again, not the best solution for the forces. I had to compromise to make something work without being ugly. The side belts will bolt to the rear triangle piece on the inner quarter panels.
Anyway, not sure if it is the best or even a working solution. I believe what I have done will keep people in the car through most of the accidents I hope never to have. Certainly there will be accidents that these will fail on, but I figure there are many accidents in this car that will have bad outcomes not matter what. Seat belts are all about probabilities.