During one of my late night research binges, I found that Geo Metro disc brakes and calipers will fit inside a 13 inch rim. Brackets would have to be fabricated... But such is 410/411 life!
http://www.fordmuscleforums.com/mustang-pages-1965-1973/527929-disk-brakes-13-wheels-yes-use-complete-geo-metro-brake-system.html#/forumsite/20490/topics/527929?page=1