Sorry to be a "downer" about something that seems relatively small and simple, but I think cutting a spring cup off another housing and welding it on your existing housing would be difficult and expensive. The shop would have to get it properly aligned (in the same plane as the other spring cup) - that may not be too difficult, but welding on the housing is likely to warp the housing. I've seen what usually has to be done to prevent housing warpage. Chassis-fab shops have fixtures to prevent warpage when narrowing housings, but the axle must be disassembled and the differential removed because part of the fixture is held by the differential carrier bearings. I don't know if chassis shops would have a fixture for that housing - the main variables are the outer bearing-race diameters for the carrier and axle bearings. (EDIT: Mittler Bros and Alston Chassisworks say their narrowing fixtures work with the Dana 60) Also, good luck finding someone who'll let you cut the pocket off a complete axle and render an otherwise complete and functional assembly unsellable. Since these axles don't have a drop-out center-section like Ford 9" axles, you may as well buy the complete axle assembly instead of having your differential assembly and gears removed and set up in another housing.
Depending on how bad it's rusted, perhaps a fab-shop could build up something to replace the rusted-out portion, but it would depend greatly on how much is left of the original mount and the complexity. You might see if you can buy a fabricated spring cup (its a Dana 60?), and see if there's a fab-shop in your area that can R&R it, but I think the complete axle is the least hassle.
