What’s this part?

Greetings! I’ve had my 64 dart for about a year now. Its a fun little car. Made a number of improvements from when I got it. Today I was doing a spit wash of the engine and engine bay with gunk and purple power. And I think I found my oil leak.

But before I post that thread... what is the name of this part that is located where the engine meets the transmission?

Thanks!

Greg