What are you going to do to arrest the rust inside and outside of the boxed sections prior to welding on caps? The caps you have can be coated with weldable sealant/paint prior to install and sealed along the welds after.
Have you looked at the US Cartool site? I would consider doing your car in a similar fashion as their body in white. Replacing the crossmember and other sheetmetal rather than capping it?
There's another similar site that has even more actual replacement components rather than capping the old ones. They specialize in rust repair parts, but I can't recall their name. Maybe someone here will. There is also AMD.