Just so you know what you are (and are not) working on: based on the location of the water pump, the engine in your photos is not a 54/55 261. The earlier 261s (you said you are a 1953) would not need a water-pump adapter plate, because the water pump would be higher on earlier 261s. The 261 came into production in 1954 (but some might have been cast in 1953?).
If you post the casting codes (and/or engine serial number), you will find out the year of your engine (if you are interested).
As suggested above, the thermostat is an easy thing to check. The condition of your radiator is the next thing to consider. The adapter plate is the next thing to deal with. And finally, a clogged block is just about the last possibility.
