Just need the tube large enough not to cover or squeeze the exhaust flow out the head. The turbo will be your biggest restriction in your exhaust. Give you an idea. Banks twin manifold hit for a small chevy is 1 7/8ths logs and have made 1500hp. Most just cant fathum that.
I just say dont go to the big side for primary tubes. Keep the velocity up to help spool. With a 499 inch your not gonna have this issue at all.
S480 has a 96mm turbine wheel and biggest i know is a 1.32ar. Your gonna get extremely quick spool with this on your engine. Gonna be really hard to control power honestly. Consider very large wastegates for sure if your running a single s480.
Id strongly suggest you look into a garret gt budget turbo based of a gt50 frame. Will have a larger 115(?)mm turbine wheel and breath easier on your big cube engine. Will allow better spool control for better power management. Anything like your doing is gonna need power management. Even low levels is gonna be monsterous power.
