I have completed a very similar project ... https://www.homebrewtalk.com/f51/motorized-grain-mills-time-show-them-off-159950/index17.html
I used a 2" on the motor and 7" on the mill. This should give about 550-600 from a 1725 motor.
(WOW, I thought my 1/2h farm duty was overkill, 3/4? Look...