Trial fit center two F-772B floor stiffeners in place and discovered an interference with the vertical center firewall stiffeners. Looking at other build logs I see that I'm not the first to have this issue. Notched back about 3/4" by 3/16" and now have good clearance. Clamped them in place & back drilled through the skin. All went well. Way back when I was building the firewall, there was a note on print 19 on the F-601J angle detail to drill 3 holes in assy with the F-772B. Looking at this, It seems like there might be an easier way. So I removed the floor skin, laid out the holes on the F-601J and dried them to #40. Then I reassembled everything and match drilled the F-772B with my 90* angle drill through the F-601J. Now that it's all clecoed together, the next time I have that skin off I can up size these holes to #30 for -4 rivets with easy access. When the two outboard stiffeners are positioned, theres nothing to clamp them to. The plans tell you that you need a helper to position these from inside while you match drill through the skin, but there are guys who have managed to do this by themselves with wood blocks and duct tape and whatnot. I messed around with this for about 1/2 hour and really couldn't figure out a good way to hole these secure by myself, so I enlisted the help of my daughter. Might as well get some free labor before she goes back to school next week! Unfortunately, we evidently managed to let this angle slip off center and I'm not happy at all with how close one of the holes is to the vertex of the angle, so I've ordered a replacement from Van's. Their shipping dept is out on Covid quarantine right now so it likely won't ship for a couple of weeks :( Fortunately, there are some small tasks that I can do to stay productive, and I'm headed to Arizona for a week on Friday anyway, so it's not the end of the world.