Ok lets get back on topic here...roof raise. I kept a very detailed log of my massive raise on my blog, http://www.fuzzytravel.com/waalner/
now I know there are probably much wiser people on this forum than I, but it may give you some other ideas. I used 1"X2" tubing then used wood to strap the sides of the bus. I used wood because as mentioned above the 1" X 2" sticks out and makes re-skinning difficult, so the strapping on the upper half is thinner than the strapping that goes against the existing metal skin on the lower half, bringing the flat plane of the side of the bus completely even. In my case I plan to re-skin the entire sides not just the upper section as I am also lowering the sides of the bus to about 10" from the ground. I will sandwich expanded polystyrene insulation between the old yellow skin on the lower half of the bus and the new flat galvanized sheets I bought for the purpose.
I wouldn't be scared about using your economical rusty tubing but I would be sure the rust is only surface rust and when you clean it use only a sanding disk on a grinder as you don't want to remove too much material. I would weld the two pieces of tubing together first then treat them as they are just one 1" X 2". I used my father-in-laws 220 welder for the frame extensions and it really penetrated the steel I have since bought a 110 lincoln electric welder and although its sufficient, (maybe using two passes at times) I feel good about seeing those two pieces of steel so absolutely one on the frame. But I too have only been welding for a year and a half now so I am most certainly not a pro!