The truck in my sig. 88 F-150 with the 351W. Immediately behind the exhaust pipe "Y" is the factory installed thing that hangs down fairly low visible below the frame on these trucks, it also has a metal shield on it.Is this a resonator or a catylitic convertor.

Behind that I'm 3 '' inch pipe back and free flow cat and flowmaster, I left the other thing in place until I figure out what it is because we are subject to emissions where I live.

 

It's a cat. Mine also has it, but I also had another, bigger cat after that one. I took the bigger one off.