One thing I always have trouble with when putting a thermostat housing on is trying to keep the thermostat seated in the housing and not letting it slip between the intake and the housing.
A few tricks I've learned:
1. Only use a thin film of RTV on both sides of the thermostat gasket.
2. Get the sealing surfaces surgically clean with a razor and some brake cleaner on a rag.
3. Use a dab of some black or hi-tack RTV to keep the thermostat seated in its groove while installing.
4. Get a thermostat gasket with an adhesive side (made just for this reason).
5. Put thread sealant on the housing bolts that go into the water.
Good luck!
Wes