These are available from several seuppliers, Speedway and Chevs of the 40's come to mind. A good metal man can fab them up fairly easily. You're prolly going to want to set the diff. up on jack stands under the car and carefully mark the locations according to spring location and pinion angle. (You can search under pinion angle here if you're not sure how to set it)

As far as the welding goes, I'd take it to a quality rod builder or certified welder and have them do it, the housing can warp under heat if not done correctly.

Good luck and keep us posted.