Since you haven't really increased the air flow into or out of the engine, 2" to 2 1/4" would be fine. 2 1/2" would be as big as I would go. The stock head and cam regulate the air flow, stock engines came with 1 7/8" or 2" pipe.
Dad's race car has a 478 cu in engine and it runs best with 3" pipe to the mufflers and 2 1/2" tail pipes. 4200 lbs and runs 11.30's at 117 mph in the 1/4!
Find you a dual inlet single outlet muffler, then you have the best of both. Duel pipes up where you need them, and single tail pie for easy and longevity. To little air flow can't keep the pipes hot enough to burn out the water and acids which eat away at the metal.
Joe
