Iron heads/iron block? Wire brush that sucker. I use a brush mounted on a 4.5" angle grinder. Takes off anything and everything on those surfaces in seconds. More often than not, the brass bristle brush is plenty enough to remove all the crud, and won't damage the steel surfaces. But I've even used a knotted wire brush on iron heads before without issue.
Rotate the crank so the piston is down in the bore about 1/2", and stuff some paper towel down there to catch the crap, but is still low enough not to be caught by the brush. Run all the gunk off, pull towel, clean up what little is left in bore, and done!
