I have a '53 235 with Powerglide engine and I believe that the water pump is the same as a 216 water pump (except for the narrower pulley which doesn't matter as long as you have the correct belt). I don't believe they changed until '55 or '56 when they went with a total redesign of the engine block and the water pump.
Don't freak yet. You have received some very good advice so far.
It is a bear to properly clean the water jackets in these engines without completely disassembling them. It is still a bear to clean them out even when they are completely disassembled.Most machine shops around today do not seem to be very familiar with this problem and it frequently gets missed.
If your thermostat does not have a small hole in it, buy one which has the "jiggle hole" in it. A good parts guy knows what that means. It alleviates the air lock issue you are likely having.
