05 Ford Mustang Collant leak

When putting the thermostat housing back on, it's easy for the thermostat to drop down out of it's recess and cause a massive leak even though it looks tight. I've done it a few times myself. I like to wiggle the housing back and forth during reassembly to feel that everything is seated correctly. Hold it in place while putting bolts back in finger tight, then torque gently/evenly. Also make sure the thermostat's coil faces the engine.