Dave-
Thanks a bunch.  Very helpful!  So how are you supposed to cartop one of these with the floor rack all jiggly, dangling from the two ends?  I guess you don't, at least not if you want an unbroken floor rack!  Just to be clear, if there are five cross pieces, then one is in the center; your center spread is 10 1/4 inches.  Is this including the extended ends of the center cross piece, or just from outside to outside of the runners?  Again- thanks very much.  I didn't expect such a thoroughly helpful answer!
H.E.