I would imagine it wouldn't be much different than machining H2 wheels to fit over ford hubs. Chances are to have it machined will be between $30 and $50 a wheel though.
I've used a hole saw (horrible) and a plasma cutter (fantastic) in the past. The plasma was the way to go. A sawzall would be really tough. I was worried about my wheel balance being off if I didn't cut it straight. You can clean up in the insides with one of these grinding stones for your angle grinder (available @ humphries in AF, I also have one you can borrow).
I would imagine it wouldn't be much different than machining H2 wheels to fit over ford hubs. Chances are to have it machined will be between $30 and $50 a wheel though.
I could cut you a template that will bolt through some stud holes, and use it as a guide to run the plasma around. That way you'd get a fairly clean, mostly circular hole....
Otherwise a big lathe would be the "right" tool for the job. It'll be a pretty big one though, to fit a wheel on. (even a 15" wheel is over 16" in diameter)
You could probably also use a wheel spacer as a template if you're ghetto.
My chevy wheels had a nice ridge/ring on them right where they needed to be cut. I can't remember if they needed to be 4", 4.25", or 4.5" in dia to clear the hubs.
I opted against cutting the wheels. This is my wife's daily driver or I probably would have tried it but I found a good wheel and tire combo to replace the 33s that were on there.