I did the fenders first, then RBs, then apron. Loosen fender if you need a little fore/aft adjustment.
Both my rear fenders had been welded to RBs by a past owner. They were also cracked around the weld, so once I repaired those, neither fender matched the exact curve in the running board. But it's not so bad you notice. Then I discovered one of my splash aprons was almost too long, (probably due to the repairs on that fender and cab corner were a little thicker than original or something--doesn't take much). In the end I was able to get adequate clearance and it looks good, so I'm happy.
I fabricated my own fender to RB gaskets out of heavy duty thick dirt bike inner tube, also helps mitigate the unmatched curve.
