After much research and consternation, I finally mixed up a little proseal and started assembling pieces of the tanks. Started with the end ribs. Ended up installing a -6 rivet in the tooling holes as others have done. Ordered the shortest I could from spruce and then shortened them a bit on the belt sander. Installed with proseal. Then installed the anti-rotation plate and the fuel bungs. I cleco'd the ribs into the skin to support them while I bucked rivets. Could have squeezed the fuel bungs but went ahead and bucked them as I was set up for it. Got it all riveted then went back to seal up each rivet. Difficult to discretely seal each rivet so slathered on a good amount on the backside of each bung to try and encapsulate everything. I put a little proseal on the nut plates for the fuel sender but will plan to encapsulate the entire nutplate and screws when the senders are installed. Contemplated installing the return line AN fitting wet while installing the anti-rotation plate, but ultimately decided not to. Just very carefully cleaned out any extra proseal that would prevent it from seating in the future. Total of 15g proseal to seal the external ribs.