68 hemi aluminum waterpump housing.

Here's what a quick web search found:

1 - Start by trying a commercial rust remover product such as Whink, CLR, naval jelly, Rust-Oxy or an organic rust remover. Apply with a cloth or a nylon scrubbing pad, rubbing to remove rust stains. These products are best for polished aluminum surfaces because they don’t contain abrasives.

2 - Use steel wool for really stubborn rust stains. Use grade four-zero to six-zero (very fine) to rub out rust stains, then wash off residue with water or mild detergent and water. Steel wool can also be used to remove the white powder that forms from oxidation of aluminum.

3 - Try a method for light rust stains that appears in many Internet sources. Ball up a piece of aluminum foil about eight inches square, with the shiny side out. Rub the foil ball over rust spots to remove surface rust.

Read more : http://www.ehow.com/how_4480346_remove-rust-from-aluminum.html



Also, have you tried WD-40? I've seen that work before to remove rust stains on aluminum.
On #3 above, I saw another suggestion to pour Coca-Cola on the aluminum foil before rubbing rust spots.
Also saw a suggestion to use SOS pads.

I haven't used any of these methods. Good luck and let us know how they work!