Heater core help

[Beachcomber]

I am trying to replace my core after a local muffler shop cracked it while removing the exhaust manifolds.
I towed the truck home after they said, "it was leaking when you came in".
I didn't give them a cent.

I had to did in the scrap pile to find 3 of the 4 heat shields off the manifolds too.

I need to know what is the black tar-like stuff to seal the box back up?

thanks