I would have a piece of curved half tubing cut to shape, and welded to the top of the crossmember (making a hump), then cut the bottom out to allow the exhaust pipe to pass through. Finally have a piece/pieces welded to fill the openings where the piece was cut out. If you approach it in that sequence, you should be able to have it done without a jig to hold the crossmember alignment. It may even be possible to complete the job while the crossmember is still in the car.
I do not know the wall thickness of the tubing, so you will have to determine what it is to duplicate the thickness.