First of all kudos to you on your car build. I have a great deal of respect for the work you have been doing on it - and thanks for sharing - lots of good ideas there!
I bought 3 point retractable seat belts for my son's 73 Road Runner, and they work well. In that installation, I didn't want the big, ugly retractor sitting out, so we took off the back panel and hid them in there. There was enough room and the belt happened to line up with ash tray hole, so we simply routed it out there.
Now, I know that is a 73 and not a 68-70. With my 70 Road Runner, I am planning to do something similar, but have not yet executed on the idea.
Generally, my thought is to again hide the retractor behind the rear seat side panel. Unlike the 73, there isn't enough room in the 70 to fit the retractor behind the panel, so I am thinking I need to cut some of the inner metal bracing on the body to make a pocket for it. If it is placed at the front part of the panel, it looks like it will line up with the roof seat belt connection point.
Finally, I would need to make a slot where the belt could pass through. At the moment, I am thinking about making a slot near the front, top of the panel and using a "Bezel Webbing Guide".
Of course, I don't know if some of the positioning ideas I have will truly work. I was just starting to think about measuring this and starting work on it - as a matter of fact I had the bezel picture up in another browser when I read your post! Either way, I figure a man of your skills may be able to run with this idea! (I also want to put 3 point retractable seat belts in the back seat as well and pass those through the package tray - but one project at a time!)
Best of luck!
Hawk