Jim Hand ran 11.80's on dual 2.5" exhaust in a 4000 lb GTO/LeMans wagon with a 7.5L 455 (bored, really was a 474 CID). 2.5" is MORE than enough for a 4L or 4.3L V8.

Keep the diameter smaller, you'll have higher flow thru the pipes when they are close to and at resonance.