Agree with Carl to leave on the frame. I also don't see "welding on your back" a problem in this case. Welding thick stiff with stringer and weave is not fun, but this kind of welding is stacked spots. So it's just Zap welding one at a time. The issues associated with upside down welding don't exist. Removing the cab seems just an option if you have the wherewithal.
Removing the cab is not so simple, lots of things to remove and it's real heavy. Plus it's nice a shady under there.
WIcruiser: Do your top side patching, then decide on how to do the under later. A whole lot can be done with out laying on your back or removal. After trying on your back or side, you will know if it's OK or not. Be sure to jack the truck up so you can sit in a chair for the initial top side welding/fitting/measuring. I have four of those old 50's office chairs as my creepers.
