Mike
Maybe I'm missing something, but would it not be much easier to sheet individual airframe parts before joining major bits together?.........fuselage before centre section is added, fin before it's added to the rear fuz etc....
When building a fuselage on a vertical or horizontal crutch as you are, it's also normal to frame up and sheet one have as far as possible before turning it over to build the 2nd side. The sheeted 1st side will be very rigid making framing up the 2nd side easy. If your unsheeted structure is already stiff enough to allow the 2nd side to be built on, then I'd suggest the basic structure is rather over built and may end up a bit heavy.
Just my 2p's worth...............(Crackin project though
)
Phil