Did you drop the speed back to idle before shutting off the ignition? If the speed is much above idle, the engine will "Diesel" from compression trying to ignite the fuel that's still being drawn through the carburetor venturi.
Drain all the coolant out, plug the water pump inlet, and pour a couple of bottles of "C.L.R." tile cleaner into the block through the thermostat housing. Let it sit for 2-3 hours, then flush with clean water and add a box of baking soda and fill with clear water. Run the engine for 10 minutes and flush again. That should do a better job of dislodging the crud in the bottom of the block than the half-donkey water jacket cleaning job your machine shop did.
Jerry
