With our cars the restriction is in the downpipe and forward section of the exhaust. The typical figure most claim for the catback is 350 WHP being where it starts to become restrictive. The only point in investing back there is sound.
3" downpipe will help with one major issue, the u-bend where the second O2 sensor is. Gets to be 2" or less IRRC. Front exhaust manifold is also pretty bad.
Edit: I see how it works now... You don't need more than 2.5" piping back there. A single 3" pipe is the proper size for a fairly built 350, not for a lightly modded 231 cu in V6
