I managed to drill the two outboard floor stiffeners by myself. The problem with these is that there is nothing to clamp them to. I ended up using some duct tape and a long stick to prop them up against the skin until I could get a hole at each end drilled. After that it was pretty straightforward.
The two center stiffeners also needed to be drilled to the cover support ribs at the aft end.
And they need to be drilled to the F-601J angles on the front end. DWG 19 specifies that these should be drilled in assembly with the floor stiffeners, but it would have been much easier if I had laid these out and at least drilled them out to #40 back when I was constructing the firewall. Oh well, I managed to get it done without too much pain.
