What thermostat?

[bigmoe]

I recently completed my swap and got it started without any problems, or so I thought. This morning I see coolant on the floor, maybe a cup fulls worth. Can't tell where but it seems to be under the water pump more on the thermostat side. The 08 5.3 only has 69,000kms and has never been apart. I should pull the water pump and put new gaskets in and maybe put a thermostat in while I'm at it. Looks like a stock thermostat is 187 degrees. Should I go stock or cooler? Longest I ran it was 15 minutes so it never quite got up to full temp and I haven't had the fan kick in yet.