The flat ribs can be partially riveted to the center section before the full assembly of the fuse. The only holes that can't be filled now are where the conical bend of the side skins attach in the aft outboard area. But - you need to be able to pry up the bottom skin to get the conical bend underneath. So that limits how many you can put in. I omitted the most aft four rivets in the inboard line and stopped riveting the outboard edge just forward of the future flap actuation hole.
I could reach everything with the squeezer with exception of the most forward hole.
