Heater box to firewall sealer?

[DirtyLarry]

So, the heater core started leaking this past summer and I finally got around to replacing it a few weeks ago but it continued to leak antifreeze on garage floor even after the new heater core was installed. I figured it the problem was the Vhatozone heater core so I grabbed another new one from Napa and went to install it tonight only to find the Vhatozone heater core was not leaking after all but rather the heater box had a lot of coolant in it from when the original heater core was leaking.

Long story short, I pulled the heater box from the firewall to clean it up (what a pain in the a$$ on an A/C cab by the way!) but now need to find something to use to seal the heater box to the fire wall. I looked through the LMC catalog and didn’t see anything. I also surfed Lowe’s website to see what they have for rope dope.

Any suggestions???