Cool Hand,
Nice photo with your own overlay of planned seat belt arrangement....much better than a description alone.
A few observations which might be worth mentioning.
All the seat belt hardware should be bolted to reinforced anchoring points on the car and not welded.
The steel straps are the "reinforced anchoring points" and will have mounting holes in them or threaded plates. The seat belts are bolted on to the flat steel side straps which are in turn welded onto the top part of the side body panel and in the back to the horizontal rear body belt reinforcing members and the vertical body panel seam.
The side straps stop before the vertical post and rely on the strength of the side body panel. All of this will be covered by upholstery. If the shoulder belt needs to be mounted higher the side straps could be extended upwards along the vertical post in an "L" shape.
Mounting the retracting devices with a regular seat belt angle bracket bolted to the frame instead of to the metal body vertical supports would provide a much stronger installation.
YUP !
When installing the retractors at the floor, the belt should come directly out of the retractor and not angled. This requires the upper shoulder bracket to be lined up directly over the retractor.
Yes, that is what is intended even though the photo may not have been clear on that. As the belts come up from the retractor to the top anchor they are free to slide through the top anchor. They are not solid at that point.
How will you install the car interior panels and trim over the projecting steel straps?
The steel straps should not be much thicker than the rolled relief panel strengtheners and I don't think the interior panels will be affected much, but I could be wrong on that.
Please take my comments as constructive and not criticism. There are many ways to skin a cat. The most important thing is that you are thinking about safety in our cars which are woefully lacking in that respect.
Good luck with your installation............Glen