The plans call for pull rivets
I am at the exact same place in construction on my 8A. My drawings call for pull rivets on those rear ribs. I mocked up everything with clecoes previously with the thought of using driven rivets and it is impossible if you use the sequence called for in the plans. The top flanges on the ribs (the ones that attach to the floor skin) are all pointed towards the inside, making it impossible to roll back the skin from the sides to buck those rivets. While it might be possible to rivet the ribs to the floor first and then rivet the belly skin on last, that is not the sequence called out in the plans. I am half way through this process. First, as per the plans, I riveted the ribs and the crosspiece (the F802UPP) to the belly skin, using proseal at the joints. Next, I prosealed the whole structure to the floor skin, using clecoes at every hole. After two days, I will remove the clecoes and rivet the crosspiece and the ribs to the floor. The crosspiece can be hard riveted because you can reach in to buck those rivets. The ribs will have to be affixed with pull rivets. That is how it is working for me. Your mileage may vary. Feel free to call if you want to discuss this further. As I said, I am right there in construction. 404-310-5128