The HP is ok and so is the motor shaft size. The speed is quite fast but step down pulleys will be necessary. What you need is a larger pulley on the grinder drive shaft and a smaller one on the motor shaft. Pulleys for the 5/8" shaft size can be a 2-3 or 4 step. The smallest beginning size step is 2", which you would want on the motor shaft. You want a 3" initial step size on the grinder drive shaft or larger. With the speed of that motor I would use a 4 step pulley system. The small pulley should be a 2-3-4-5" and the large pulley a 3-4-5-6"
Pay attention to the direction of rotation on the motor. You will want a CCW ( counter clockwise) rotation.
The groove in the shaft is called a "keyway". The pulleys will also have this. A 3/16" square, 1" long piece of mild steel is placed in the keyways to lock everything together. The pulleys should also have a set screw built in. This screws down onto the "key". I sould have my grinders here at home in a week or two. Stop by and look them over if you want to. Building them is a bit of a learning curve.