Flushing a engine block that has been sitting.

[Big Block Bill]

The only way we have ever had ANY success with a condition like yours, is to knock out ALL the freeze plugs we can get at, pull all the coolant drain plugs, and use brass gun cleaning brushes to clear out what we can get at. Using an after market high flow / volumne pump is just going to "Band Aid" the real problem, and when any "Chunks" of old deposits come loose, they will just get clogged in the radiator core. Zep used to make a product called "Zeptine". It was an industrial cleaner for heat exchangers. Good luck, and where an old raincoat!

Bill