The trick to getting the side "skirts" to fit is after hooking the stringers into the bulkhead cutout, cleco the bulkhead to the side skirt starting with the most bottom hole (the tailcone is flipped upside down so, really, it's the uppermost hole.

After I got them to fit, I took them apart and deburred every hole after using the reamer to make the holes the proper size.

Then I reassembled, clecoed, and riveted the left and and right bottom skins together and riveted the bulkheads to the bottom skin before running out of time to rivet the side "skirts."

Next time.