How do you intend to use the monopod?
Do you intend to use it with any other lenses or just the 50-500?
How heavy is the 50-500?
It could be that NO head on the monopod would be the most suitable setup, depending on how you anticipate using it. If you are going to 1) use only the 50-500 and 2) are just planning on the classic vertical monopod position with you standing vertically behind it and 3) are planning on always shooting with the lens roughly parallel to the ground, the tripod ring on the lens may provide all the rotational movement you need.
That said, I use a 486RC2 on my 680B monopod. However, I use it with a variety of lenses - one with a tripod ring and the others without. I use it in a variety of positions - standing, sitting, etc., with the shaft of the monopod at many angles to the axis of the lens.