Exhaust size for small block?

I didn't catch what carb you're running or what kind of ¼ times you're looking for. Some may say you should run a 750 cfm, but I'm inclined to recommend something in the 600-650 cfm range with an air valve secondary for use with an automatic. The smaller carb will definitely give you better throttle response and the vacuum secondaries won't open up until the engine can use the backside of the carb.

You may want to consider 3" to the muffer and 2½" to the outlet on your exhaust. The smaller tail pipe will help keep exhaust velocities up, which will help with scavenging off the engine a little. (Or a 2½ - 2¼ if 3" is too big)