Exhaust pipe size

I Would Get The Headers That You Plan To Run W/360 And Put Them On And Have Your Exhaust System Made. This Will Also Help Out The 318 When You Do The 4bbl. You Can Get Good Headers From Summit For Around 100 Bucks(the Summit Brand). Exhaust And Headers Are A Pain In The Rear Do It Once And Get It Over With.as For Size Deffintly 2.50