Radiator question

New information- I was so busy trying to record all the places to shoot the engine with my inferred gun I missed the following. So, let me back up a little. Before I purchased the 1965 Dart 6cyl 225 the person I bought the car from found it in a barn. He indicated he got the car running, however the engine would over heat. He swapped out engines and put in an engine from 1969. When I purchased the car I had the original engine rebuilt professionally. After installing the engine it would run hot by the gauge very hot. the engine seems to idle okay but at highway speeds the gauge almost pegs the gauge. Now I have always contended that the car never lost antifreeze however I am incorrect. Every time I've taken it out I have had to add about a 1/2 quart of antifreeze so it must be blowing by the overflow and since there isn't an overflow bypass container I didn't realize it lost antifreeze. The car must be getting so hot it is boiling out and since I never saw steam or such and the infrequent driving of the car I didn't notice it was losing antifreeze. So this is what I have done so far:

radiator serviced- working fine
new water pump- put in an eight blade
new thermostat installed 160 degrees
new cap

is there anything I could have missed. I've received a lot of good advice but now that I realize I'm losing antifreeze I'm kind of thinking during the rebuilding process they might have missed something in cleaning the block and water jackets, is that possible. If so can I clean the block and water jackets without taking the engine out of the car? is there a flush system?