That's not right. Almost all the manual boxes are the same. The large sector was pretty much just used in cars like the large C-bodies and trucks. Your stock pitman will fit - as long as you had manual steering to begin with. the power won't swap with the manual. The only reason that the 73 and up arms changed was due to the way they mount to the center link - nothing changed with the way they fit on the steering box. Flaming River says it's the small sector.
Bottom line, if your 72 is manual now, it will work w/ the flaming river box. If you are converting from power to manual, you need a new manual arm.