The pictures at
http://www.gotbelts.com/1931model.html show how the outside units with the retractors installed are mounted (see pic above) but don't show how the center belts are mounted.
The MAFCA article
http://www.mafca.com/downloads/Technical/seat_belts_2.pdf appears to show the metal angle attached to the back of the seat unit. This is wood and I'm not sure that is a secure enough mounting point. Also, with two 175 pound occupants, that's 350 pounds times how many G's - can the seat be pulled completely out of the track in an accident?
Maybe bolting them to the floor in the trunk and tying that into a body rail somehow, then cutting a slot in the sheet metal for the belts to slide through?
Not sure yet.
Steve