I'm in the exact same process right now. Trying to decide how to replace the tube, I had some dom scrap laying around, it's the size OD as the existing cross member. Initially I thought I would be able to slide it right through, but that's not going to happen with out drilling out the section that runs through the rails, so I think I'm just going to plat the ends of the new tube, and weld the plates on the inside of the frame rails.