The Pros do these kind of repairs all the time. Take a look at the Iron Chief site. There is a good project on that site. The idea is to leave as much in place as you replace bit by bit. On some of these sites they do all the welding and then send the cab out to be stripped. That way it doesn't change shape in the process.
I am doing my cab right now and when things get a bit tough, I take a break for a while until I figure out what went wrong. It is very gratifying to see the new metal come together.
