ok i just put in my new motor, it got a b18 head with b16 bottom , i go look under my car and i got a huge gap from from cat to the header i even bought a cat for a gsr because my old one was shorter and it still is shorter what can i do to fix this?? thanks!

 

I had the same problem I just had my friend "fab" a pipe to fit the gap with a bung for the o2 sensor. Driving it to him open header sucked though.