Your pto gen is likely set for an input speed of 540, meaning you need to run your tractor's pto at that speed. Your best hp:rpm ratio is going to be with your pto set on it's lowest of the 3 levels, if you have the 3spd pto. Yes.. you could set for a higher pto speed, and run engine at lower rpm.. but you would not be getting full use of your tractor hp.
If the electrical load was super low.. only a fraction o fthe pto gens output and your tractor hp range.. then that should be ok, and save bit of wear and tear and noise.. but otherwise.. just run 540.. etc..
Soundguy