Count the teeth on your current starter bendix gear. May have 9 or 10. The 10 tooth repair works for some people. If you already have a 10 tooth bendix, the problem is elsewhere.
Couple other things to check include the bolts - are they really loose in the holes, are the threads in the holes damaged, bolts the right length, have they been drilled through all the way? Lots of potential for damage there. Check the block plate between the engine & trans as well, the hole in that aligns the starter to the flywheel. If that's egged out, you will have trouble. If it appears to be misaligned with the transmission or starter mounting holes, you will have trouble.
IMO shimming the starter is not the fix. I've never had a Ford starter that needed to be shimmed although shims do exist... the problem is just worn out old junk and crappy rebuilt starters.