When to go bigger?????

My old 3" H-pipe setup no longer fits after the 6 spd. goes in so I figured wth and am going with a 3.5" Pypes X-pipe kit and some 16" x 3.5" Borlas I scored off of Moparts. Probably will put some 2.5" turndowns on it to tame it down a bit. Plan on a bigger engine in the future so figured might as well design this exhaust for that instead wishing I did later.