About the ONLY way to CLEAN the inside water jacket area is to take a couple "Freeze Plugs" out and then prod and flush the crud loose..
" Make sure you do not have the "HOT Thermostat in it. See if you can get one with a lower temp...NAPA will be able to match it up..
Always test a thermostat in Hot water with a thermometer, to know when it does start to open..
With a GOOD water temp gauge, you can see when the thermostat opens each and every time it opens..( the water temp drops about 20 degrees).