Another thing i forgot to mention...
Only use one rivnut to secure the joint between the side panel and rear tub, and make sure its quite low ie below the line of the rear arch. Youll probably notice that they will try and "flop" away from the chassis at the top. But when you put your rear arches on, the arches and arch fixings will push the sides of the tub and sidepanel against the chassis. That way you end up without any unsightly fixings on the visible part of the join if that makes sense.
I actually cocked this one up a tiny bit its not 100% perfect but you only see to top inch or so of the join once the arches are on and its the only pic i could find!