thermostat housing

I want to replace the thermostat and housing, do I need to drain the coolant completely or since they are both at the top of the block can I get away with replacement and minimal coolant leakage. I have a full coolant flush along with other fluid replacement scheduled next week and would like...

I'm looking for the l wiring pigtail connector for the Temperature Sensor on the Thermostat housing on a 1984 Monte Carlo SS 305 4Bbl

Might sound like a dumb question but will a leaking thermostat housing cause my regal to over heat?