Teflon tape pipe thread sealer FTW. But loose heater hoses wont be a "cause" of overheating unless you are already way low on coolant. I don't know anything about the XAT kit, but some heater hose kits have 1-way check valves in them - which would keep coolant from circulating if installed backwards, and would cause overheating.

Check and see if you are getting heat in the car, to see if the heater core is getting hot engine coolant. After that, check the thermostat (simple but effective). -JinX