68 Dart Heater Core Replacement

To replace the heater core is there some kind of gasket or sealant behind the heater core. I've been told that when I replace it i need to reseal it. I thought I only needed to pop the clips off the front of the heater box, remove 4 phillips screws and slide it out to remove. To replace it I just push it straight in replace the 4 phillips screws put the front of the heater box on and replace the clips. Am I missing something?