what ever thermostat you use check it first in hot water and a thermometer to see at what temp it opens, even new ones fail, CHEEP insurence if you think about it. If you overheat the engine vary likely your looking at a warped head. **** luck Gary Portland Ore.
If for some reason they give you the wrong thermostat (and it wouldnt be the first time trust me), then dont put the old one back in. It is better to run arround with no thermostat than to have the old blocked one in their causing flow problems.
Its no big deal running without a thermostat, it just means that your engine will take ten minutes longer to warm up, so youl need the choke out a bit longer.