Most diff castings weld fairly nicely w/ just a MIG. That doesn't mean you get the penetration or strength, just that they weld nicely.
Design your mount to have a lot of weld seam length. Avoid getting the casting hot in only one area, the differential cooling can easily pull the fresh bead out of the cast metal. Though a little preheat is a good idea. If you can lay your hand on it, but don't want to leave it there very long then that's about right. Weld a bit, let it cool. Repeat. If it is cast iron you'll want to peen the weld bead as it cools. A chipping hammer seems to work very well for this.
If you are absolutly certain that you're dealing with cast iron then Ni-rod, Rockmount 'Jupiter', or brazing are probably your best bet. I remain unconvinved that Dana hsgs, in particular, are cast iron. I believe them to be cast steel or more likely cast semi-steel. That the steel tubes are welded into it tells me that it's a production-wise weldable alloy of some sort, and cast iron wouldn't normally fall in that catagory. The difference is a rather small difference in total carbon content, but it makes a large difference in how it wants to be welded. Cast steels weld like any other steel.
