I deal with PTO's and hydraulic pumps everyday, I'm just glad that someone finally asked a question that I had the answer to.
With a MUNCIE PTO, the ratios are from an 04 ratio that will give you 28% of the engine rpm, up to an 18 ratio that will give you 154% of the engine rpm.
So if 600 rpm is what you roughly require, than a pto shouldn't be a problem. The duty cycle of the generator will be a concern, you'll probably have to put an oil cooler kit on.
Any local distributor of Muncie or Chelsea pto's should be able to gear you up with what you'll need.