HFCs + Stillen CBE is definitely a raspy combo. I remember way back when 2theextreme had this setup and his car was raspy like no other.
Regarding the HFC/Header connection. You might not need a metal gasket but you should use some sort of gasket material. Tony from Fast Intentions recommends the use of high-temp RTV silicone at that connection.
I'd definitely listen a little closer with the car at idle and with the hood up. See if you can pick up on leaking sounds at that connection. Have someone slowly rev the car for you, too.
That said, I think it could just be normal. Stillen + HFC is pretty raspy.
FF to 0:55