Recently URO Parts released an aluminum 3.6 Thermostat housing/pipe its about $60 on ECS and RockAuto. I installed one on a friends 3.6 and had no issues. I cleaned up some of the casting slag but it was minor and probably me being overly anal. The price of the part is about what the OEM plastic pipe costs.
Here are the part numbers for the other parts that touch the coolant pipe that I'd recommend changing at the same time:
Here are the part numbers for the other parts that touch the coolant pipe that I'd recommend changing at the same time: