I have the 307 that makes 200hp, but I want to do a lot to it. I was thinking of: msd ignition kit, headers, definitely dual exhaust, rebuild kit with forged pistons, and either an edelbrock pro-flo fuel injection system or the 800eps carb....plus possibly stroking it with a 350 crank
On a several small block Novas, I have ran 2 1/2" to the mufflers and 2 1/4" over the rear axle. The exhaust gases cool considerably by the time they exit the muffler and do not need as much volume for the rest of the trip. Also, there is a noticeable difference in noise between 2 1/4" & 2 1/2" tailpipes.