68 hemi aluminum waterpump housing.

Many products say "don't use on aluminum" so be careful. Citric acid works well on similar deposits in radiators, and I think is safe for aluminum. Active ingredient in Prestone SuperFlush. I bought 5 lbs on ebay (cheaper). Leave soaking in the sun since heat helps. You could let your engine do the work, just install the pump, then flush in place. I always put a screen inside the top hosee until I have flushed all the crud.