Best rust solution?

[Old Henry]

I just removed these water pumps that had sprung a leak. As far as I know these were clean when my engine was rebuilt in June.



This is after just 5 months of running plain water and two bottles of Hy-per Lube Super Coolant that claims to prevent this kind of thing.

I'm assuming that my entire block looks this bad inside so would like your best ideas of how to dissolve this rust and I'll try some of the most popular ideas on this stuff and see what happens.