I used -4 rivets at the thinest part of the flange -4.5 and -5 at the thick parts of the flange and back rivetted all of them. Came out okay. I have a couple of slightly proud rivets. I drilled out two and replaced them, but made one of them worse, so no more of that. I've got Proseal all around, under and on the shop heads so this should be leak proof. I think the slightly proud rivets are due to not getting the flange flat on the back plate due to the slight curve in the skin and fuel cap flange. Or it might have something to do with the Proseal and dimples.
3 lengths of rivets to accommodate flange
Rivetted and Proseal on flange
Flush side is okay, not great due to slightly proud rivets
