2.5'' on 318?

Flow master put out a general guide some time ago. They recomend 2-1/2 pipe for a 300 HP & up engine.
I also currently have 2-1/2 pipes on my 318.
Stock pistons in a '79 mill with Hooker headers, small crane cam and 4bbl. (3.21 gears on a stock tire) So far, it feels good.