Sorry but your information is sketchy and I'm not really understanding exactly what's going on. You acquired this Aveo some (unknown) time ago, and it sounds like you identified a leak at the t-stat housing. I will tell you that a PO installed the metal housing, because all '05 Aveos came from the factory with a plastic housing. So if those bolts had been installed loose back then, then I can't imagine how it wouldn't have been overheating regularly from the time the t-stat replacement was done, because it would never have been able to hold the proper coolant level.

And I'm also not clear on the part about threading the bolts in further with the housing removed. Beside threading the bolts in further, did you try to see if they would hold torque using spacers similar to the housing depth? If not, you need to make sure the threads are not stripped. But perhaps you've already done that and just didn't mention it.

The answer to your question is that it's highly unlikely the metal t-stat housing needs to be modified, because these parts basically all come from one or two factories and have little variation. Bottom line is that I'm not at all clear exactly what the problem is, so it would be helpful if you added more detail and filled in some of the blanks.