[Hobbiest]

Pipelines don't have to worry about being straight and true, rear axle housings do. I would suggest you take the cast iron route in regards to welding. When you weld cast iron (on anything that can have no leaks), if it is too hot to hold your hand to it, then you should stop welding until it cools. I would suggest no more than an inch bead at a time. Unfortunately, everywhere you stop, you will most likely have a small leak. the only way to remedy this, is to grind the beginning and end of the beads, and start back from the edges, continuing into the new beads. I have spent a lot of time welding cast iron (engine blocks, water pumps, etc), gas tanks, and have even done a few rear ends myself, and can tell you that unfortunately warpage is going to occur. The aformentioned method of being able to tell where the housing is out, works very well. Just remember that heat pulls the metal together (shrinks it). Good luck. If it is too warped for you to figure out how to straighten it, take it to a decent welding shop. I have found that it is the old guys who are best with this kind of thing, and the ones who work on a lot of farm equipment are especially good for some reason.