Phosphoric acid is actually readily available. The big box hardware stores carry it in the paint department, It's called Ospho.
LINKI had basically the same greasy sludge you had. After mechanically scraping it out, I used a pressure washer, then after installing the core plugs, filled the block with commercially available coolant system cleaner, which I believe contains Trisodium Phosphate. Let that set in there for a couple days and rinse it out again. Heat helps, but I didn't have a way to do that, and it seemed to do pretty well.
