When we drilled the holes for the cabin frame doublers and then went to rivet, we saw the err in our way. We had placed the holes too close to the flange on the L-angle making it impossible to get our rivet gun in from the bottom to rivet. An option was to rivet from the inside out, but that would look pretty silly and theoretically increase drag with all the rivets sticking out on the bottom side of the fuselage.
We ordered new longerons.
It took quite some time for them to arrive, USPS is very slow right now. Anyway, since that time we had learned we could grind down the head on our rivet gun a little, so before redoing the longerons we decided to see if our new, ground down rivet gun would do the job. Not quite, but just a tad more grinding and all worked fine.
So, we have two brand new cabin longerons, untouched, but we’re glad we didn’t have to redo the work.
With this step complete, we now have zero cleco’s on the bottom of the cabin frame and only a few in the tail.