Ken,
With RG-8 size coax I'd make the diameter of the coils
larger than 2 - 4 inches. A small diameter can make the
center conductor migrate to the 'outside' of the coil and
eventually short out (may take awhile). Something like a
3 pound coffe can makes a good form for RG-8. If you have
to force the bend, the diameter is too small.
If you tune the antenna for 40 meters it won't work as
well on 60 meters. The difference may not be all that much
though. I'd make it resonant on whichever band I planned
to use the most, and then 'put up' with it's performance on
other bands.
Something else you might consider. Extend the antenna the
rest of the way around the building and make it into a loop.
That should give you 80 meters and maybe even 160 meters...
'Doc