That's a good idea. I use the same method to remove bushings and bearing races. My only consern about using a bushing is the stress rise would move farther out on the tube. If I don't retube it I think the best chance is to gusset it really well. I'm gonna have to take lots of time in the prep work though or it won't come out straight. I've been thinking about how to gusset it right. I'm probably going to gusset all four locations(top, bottom, front, back) or leave the front one off.
All that said, I won't be able to make a true decision until it is on the table. The casting could be cracked out and I can't see it with out a thorough cleaning.