I suspect you're over thinking it.... Clean off the gasket material. Drop the new thermostat in boiling water. Wait till it fully expands. VERY CAREFULLY remove it from the boiling water and drop it into place and see how it fits. I think it should be nicely centered and drop in smoothly. If it doesn't then pry out the old block off plate and tap in the new block off plate, then repeat. No sense in buying more new parts until you confirmed what you have is an issue.