The conventional wisdom here is that you need to use the Ford or IH thermostat. Many or maybe even all aftermarket stats are unacceptable because they have a bypass...which may be good in other applications but the 6.9 and 7.3 blocks already have a bypass internal to the block...doubling up on it can lead to cool temps on the coolant gauge but it is deceptive because the heads can develop hot spots and it effectively becomes like running the engine without a stat. In theory a thermostat with no bypass and the stock opening temp should be ok but no one really has wanted to risk engine damage to test it or save a few bucks. I think perhaps a couple folks have run the Stant thermostats but they do seem to behave differently than the factory ones.