If you have changed radiator, pump, and other things that need or cause a refrigerant loss (water, antifreeze or whatever you are using) the engine does not get all the water in when you refill the radiator, it means AIR GETS TRAPPED INSIDE the engine mainly on the motor head where the sensor (the one in the back of the engine at the driver side is located) has to read the temp, then it will read much higher HEAD temps not water temp, thats why you get a 260º reading and the engine is not boiling.
You have to purge the air from the engine, locate the temp sensor (driver side, at the back, in the head) unplug the connector, use a deep socket wrench and remove the sensor, fill with water until all air bubbles are out then reinstall the sensor, do not overtighten or you will break it , reconect the sensor and I'm sure your problem will be solved.
I went through the same problem and thats the solution that these folks gave me and IT WORKED.
Good luck
SACEM