First, I would try an impact screwdriver. It looks like a solid bar of steel with a fitting for screwdriver blades or sockets. The mechanism is internally geared so that when you hit the end with a hammer, it rotates the blade. Auto parts stores usually have them in stock ... they're used a lot on motorcycles. Another idea would be to try some heat ... very, VERY carefully. Aluminim has a much higher coefficient of thermal expansion than steel. Be careful so that you don't harm the paint, or the temper of the aluminum.
