You could also try freezing it. Because aluminum expands and contracts at a greater rate than steel, you may have some luck.

Something else to try would be to start drilling it out, using progressively larger bits until you can fit a hacksaw blade in there. Then carefully start cutting it. A couple good whacks with a hammer and drift from the backside should take care of it.

EDIT: You could also try heating it up with a torch or heat gun. In working on cars in the past, sometimes just an application of heat and subsequent metal expansion will break whatever bond is between two frozen parts.