yea I've done TONS of welding laying upside down under trucks, I have a bald spot from doing that, caught my hair on fire.
If I were to fully box a frame, I'd most likely just pull the frame rails using loads of jack stands and duplicate them out of tubing, maybe stainless. However if I felt compelled to box the original frame I'd probably lay underneath and TIG weld it, tape the foot pedal to a solid cinder block and get a pillow or two and clean the **** out of the frame and use a good Damascus steel, quite tough and ductile, better than the original frame. Or in the case of what you did I might machine out a solid block to fill the channel in those two locations, excessive but I have the tools and materials available to me. \
Actually now that I think of it, I know where an old frame from a bronco is thats kinda rusty but it's strait should be good for a pattern, sadly I doubt anybody would be interested in a super excessively heavy duty frame unless they managed to bend theirs in an accident and that takes one **** of an impact, they'd be more worried about paying down medical bills.