I would be sure to cut out a section where there is enough length of the same dimension channel. That way you can butt the two pieces back together and weld them, then put a plate on the inside and completely weld it too. I wouldn't even be opposed to a plate welded on the outside either. If you're not sure about your welding skills, get someone else to do it. This isn't a place to skimp or let macho pride get in the way!
In short, measure twice and cut once. Good luck!
