Here is the procedure to make that repair. Hope all goes well
Removal and Installation
NOTE: If a heater core leak is suspected, the heater core must be leak-tested before it is removed from the vehicle. For additional information, refer to Section 412-00.
CAUTION: Do not attempt to bend any part of the panel/floor door lever. It is brittle and will break.
CAUTION: The heater core cover must be raised vertically before removal to avoid damage to the heater core housing.
CAUTION: Position the temperature blend door manually to correctly align the actuator and the door. Do not power the actuator electrically. If it is not engaged with the temperature blend door, damage to the actuator can occur.
NOTE: Add gasket between housing and cover before installing cover.
6.0L diesel vehicles
NOTE: Carry out Steps 11 through 13 only if installing a new heater core due to a leak.