I believe the 164 tooth is for the older models...If you have a late model mustang (Fox and later) you need a 157 tooth for the stock starter...however if you have a starter made for 164 tooth...you'll need that one.
I am running a PA SFI bell housing. depends on what size convertor you are running. i believe the PA bel housing will fit both sizes but if you have a 11" or smaller convertor then you are definately a 157 tooth. I am running a 9.5" Convertor and a 157 tooth flexplate. The starter should not be an issue. My aftermarket starter fits both 157 and 164 tooth flexplate so I bleive it is all on convertor size...
when i went to start my car the starter just spun didnt engauge with the 157t flywheel? im gonna try a 164t im not sure if the converter bolts will line up or not