Some of the older spindles are cast steel, not cast iron. Those can be welded to using normal proceedures.
Cast iron is a bit different gig. I would hesitate to advise anyone not familiar with the process to try it unsupervised.
Kreg, call the Rockmount rep and get him/her to give you some Jupiter rod to try. WAY, WAY better than stainless rod for cast iron. It's alloying falls in the 'high nickel' rod catagory, but it is specifically made for welding on cast iron.
My spindles have been extended, and you all know they've taken a pretty serious beating (a lot worse than the pics dankster posted, lol) these things have held up. They've obviously not stock, so what are they made out of?
Those are castings. They are possibly steel but more likely iron. Whoever modified these kept the stock steering arm intact instead of chopping it off. The extension on top also uses the original mounting point. Doing this significantly reduces dependency and stress on the weld joints.
They had to be heat treated to be able to be sucessfully welded when they were extended. I am sure I had beat my truck pretty hard that day(Pole Line at 50mph and no slowing down) I did jump it quite a bit that morning but the jump that broke it wasn't the biggest jump of the day, but my last. Maybe you have beat up those spindles, but like he said your steering arm hasn't been hacked off and relocated, so it isn't depending on the welds to keep it there. I am sure heat treatment was applied to your kit, since your fabricator prolly knew what he was doing