1) unless you are 100% totally sure that no flameable gas or fluid of any sort, ever, has been in those pipes, get the pipe out of the car and fill it with sand/soil/flour or similar before you put any sort of flame near it. pipes exploding ruins a day and requires new underpants.
2) Welding is probably a bit too much effort - easy to blow out the pipe. I strongly suggest you use an aluminium brazing method instead. Most are fine with a regular butane flame ( actually - work better with regular butane ), and the brazing alloy will be stronger than the aluminium base layer.
Al Brazing with these rods is entirely black and white - it either works or it doesn't. the difference is how you prepare the metal. Clear up as normal, degrease etc, but the LAST act you do, before you put a flame to it, is scrub the area with a stainless steel wire brush. If you don't brush it, it won't work. If you use a brass brush, it won't work. Use a regular steel brush, it won't work. Use a stainless brush, it'll work perfectly.