You can also use citric acid. You can get about 2 pounds of it (dry) on Amazon for $12. But either citric acid or phosphoric acid will both damage metal if the mix is too strong and if it is left too long. CLR can be used. Remove your water pump, drain the block, close drain holes and fill as full as possible. Leave overnight. Repeat if needed. CLR tends to lose its "oomph" over time.

The advantage to citric acid is you can use what you don't need in the engine as a house cleaner. Mix with tap water to a 6% strength and use to clean toilets, bathtubs, lavatories and stainless steel sinks in a spray bottle. It is extremely good for removing deposits from shower heads, faucets, etc. Makes them look new again. Add a wee bit of liquid soap like Dawn detergent for added surfactant ability but do not use this on granite, natural stone or colored grout. Good luck!

~ Jon
1952 1/2 ton with 1959 235 | T5 with 3.07 rear end