The reduction from 75 mm to 63 mm will slow it down by 16%, which should be a quite noticeable difference. If it is still too fast for comfort and safety, just switch to the 50 mm one which will give you an additional 20% reduction in speed. There should be no difficulty in getting the outcome you want, and you will continue to have the choices available if you change your preference later. The belt drive is very convenient in that respect.
