I too used the Juliano's 3 point belts in my 52 GMC and 50 Chevy trucks but did not feel comfortable in snaking the fixed part of the system through the seat cushions and came up with a more direct "pull" by using some angle iron brackets mounted to the floor.
bracket I had to cut out a piece of the back of the seat frame for the belt to pass through and used pieces of a plastic binder spline as an anti-chafe guard.
cutout While I welded in my upper attachment point for the over-the-shoulder attachment point I don't think it is absolutely necessary as the plate is also held in place with a bolt. I think all that would be necessary is to secure it temporarily until that bolt is in place.
