The ratio of the gearbox will determine the rpm of your stump grinder. I believe someone just built one using a gearbox. How big a wheel, and how many teeth will you be using?
With a 3:1 gear ratio, the 540 rpm will give 1620 rpm on the output shaft. I am not sure what cutting tip velocity should be, but some of the stump grinding folks will probably chime in and enlighten us.