This housing is often corroded and when you change your thermostat, it's great to have a new one on hand. The flat top version is for the 948 engine. The round version is for the 1275 engine (works on a 1098, too). 

If you are changing from a cross flow radiator to a down flow original type (or aluminum) radiator, you will need a round top. 

When replacing the housing, it's common for the studs to be damaged, we

If you have the housing off, you will need as gasket, and it makes sense to replace the thermostat, we with both!