3/8 or 6an fuel line max hp

Don't think you can correlate fuel line size to horsepower exactly.

To answer the question though, I would think once you start getting past 450-475hp you're probably going to want a 1/2 inch line just to keep the volume up.

Are you just thinking out loud or planning a build? What are you doing with the car? What's the rest of the fuel system set up?