Roof vent question?

I am installing a maxxfan vent on the roof of my '06 2500.There is one rib in the roof that holds the mounting flange at the rear up off the roof .Some where I have seen how this situation was handled but I can't find the pictures now.Any one know the link or have some experiance with this.
Thanks Tom

You mean this one...



...from here...

http://www.steynfamily.com/mercedes-sprinter-van-conversion-to-mercedes-sprinter-van.htm#2

I seem to recall someone cutting and flatening the rib.There is only one that needs to be fixed.It is at the back edge of the vent in the center.I don't want to build up the flange just for that one ridge if at all possible.
Thanks for the help folks!