I would put the largest true dual pipes you can fit and run restrictive reverse flow mufflers. I mention "restrictive" because you need some backpressure to maintain torque. It's a similar setup that I had on my '86 Ram 4x4 318 2bbl with 3.23's and there was a noticible increase. If I remember correct it was 2 1/2" to the mufflers and 2 3/4" out the back. Smaller displacement and heavier vehicle from what you have........

Otherwise, if you are really adventurous, a better solution would be a 360. There's no substitute for cubic inches and if you use the Magnum barrel intake, it builds alot of torque in the 1500 to 1800 range. Sure, it would cost more money but I venture a guess that a 1/4" difference in pipe isn't going to make any noticible increase and could be a waste of money.