[Andy A.]

Also, I believe that the answer given by the transmission designers was that the output shaft was 16mm because it was the most cost effective method, while still giving teams the option of using 5/8ths sprockets with relatively little effort.

Reaming out sprockets is fairly easy. If you can not do this, then consider turning down the output shaft either by cutting on lathe or by turning it and liberal use of sandpaper. None of the methods of using 5/8ths hardware is ideal, so you should consider buying sprockets from IFI through their website. You are stuck with 21 teeth from IFI. Another option is to look around and find a 16mm sprocket supply. They are out there.

-Andy A.