So, you want to reduce the speed to 175-200 or so RPM for milling.
Source (motor) - 1725
Desired (mill) - 200
1725 / 200 = 8.625
so, a ration of 8.625:1
If you use a 2" diameter v-belt sheave on your motor (pretty common), you will need one 8.625 times larger, or 17.25" diameter on your mill drive shaft.
Then, figure out how long of a belt you need to make this work, guessing 40-60 inches would do the trick. 60" belt would be 12" gap from center of your motor shaft to center of your mill shaft.
Hope this helps! Good luck!