As shown, I'm heating up the nut, to expand it away from the stud, breaking the rust bond. What's not shown, is that immediately after heating, I touch the end of an old (wax) candle to the exposed threads. The wax is drawn towards the heat, and wicks its way into the threads, lubricating them. At this point, the nut is easily backed off. Backing and reversing gradually, and reheating if required, until it's all the way off.
In your case, you'll want to heat the corner of the cast iron flange, to expand it away from the stud (you don't want the stud expanding making a tighter bond). Sometimes, just the act of heating and (natural) cooling is enough to break the bond. In any case, try the wax, I think you'll be surprised.
As for the Vise-Grips versus double-nuts, since you're already replacing the studs anyway, who cares? The Vise-Grips will be a lot faster, and what I'd use if I wasn't out for saving the studs.