JB Weld all the way!!
seriously. That is what a couple of welding shops recommended me to do. I called about 10 shops in my area and no one would weld the flange on for me. It has something to do with the fact that the aluminum is porous with oil or something like that. I don't really understand it though.
Anyway, chris, you are going to have to take off your oil pan. And if you can't get the flange welded onto the pan, then use JB Weld. That's what I used. It's only a couple bucks and it is as hard as a rock. That shit is the shit!:P
later,
aj