Yes - that's how mine is mounted. Belt anchor point first (against the inner rocker), then the retractor itself, with the same mounting bolt on each side. See my picture on the previous page for the finished product (apologies for not taking more pics when I installed the kit - I thought it was one of the more straight-forward installations, actually). The shape of the belt anchor keeps it wedged firmly against the inner rocker, and the bolt being mounted in a factory reinforced mounting point - the extra hole in the floor is not necessary.
Above and slightly to the rear of the front edge of the driver & passenger quarter windows are the factory upper shoulder belt anchor points for the pivoting belt loop to be secured. It's just above the interior door trim and might be hidden under the headliner (if it was replaced and the shoulder belts not reinstalled).
This pic (from an article on Mustang 360 on how to install the headliner), shows the factory upper shoulder belts in-place. The rear-most mounting point is where the shoulder belt anchored and would droop down from there. That is where the pivoting belt loop (for lack of better term) is fastened.
This pic shows what I believe to be the actual mounting point itself. Toward the very top of the pic is a bolt hole that should be the mounting point for the upper shoulder belt anchor.
When I installed my headliner, I made sure to put all of the fasteners I would be needing (including the seat belt anchor bolts) where they were supposed to go, unscrewed maybe a couple of threads. Then, when it was time to punch the holes into the headliner for the fasteners (like the trim pieces, coat hooks, belt anchors, sun visors, etc.), I could feel them through the headliner and knew exactly where to cut.
I saw your installation pics J, and you did a nice job - it looks very cleanly done. ::thumb:: But if CJPP would've had the right instructions for '71-'73 owners, it would've saved you a lot of hassle. The other no-name belt kit they offer for the '68-'73s has a different set of mounting instructions as well.
Hope this helps.