I think you are right. The weight of your lens and camera will exceeds max weight load capacity of this monopod. Some time ago I made a mistake when I used Manfrotto head for monopod with low weight capacity and it broke eventually. I was lucky, because camera with lens had fallen into my hands.
Yeah, I wont risk that - I think I'll have to get a better monopod. What do you people think of a Manfrotto 681B?