Get a gallon of phosphoric acid. Close the freeze plugs and the hole where the water pump sits.

Mix the acid with water until you have enough to fill the block (and the head later).

Let it sit over night.

The phosphoric acid ( the same stuff like in coke by the way) not only cleans out all the gunk it leaves a surface that prevents rost.

Drain the block and flush it with a garden hose or better a steam blaster for half an hour.

Do it outside or in a well vented room because it stinks awfull. Use glasses and wear gloves. The acid will not burn a hole in your hand but it is nasty stuff.

This is - by the way - the same thing what this company does for 80$.