TMP - I don't believe you will have to take the underdash heater box out to get to the fan. You may want to for other reasons, but not for the fan. Look close at the fan housing that sticks out into the engine bay. There should be 3 small sheet metal screws. Remove those and that outer ring will come out which allows removal of the fan.
On assembly, that outer ring is attached to the ring shown in your pics above and the fan & motor attaches as a unit from the engine bay.
C