I'm running a 632ci with two stages. 400 hp on 1st 300 hp on second. I use 23/8 Hooker Headers with a 12" merge collector 4.5" diam. No probs. As long as your pipes are bigger than your exhaust ports its all good. I think the collector is the most important part, making back pressure etc.
Here's what Hookers say:
By altering the relationship between the primary length and collector diameter and length it is possible to increase or decrease the torque and horsepower output at any specific R.P.M. level. For a slick track the header primaries or Collectors can be shortened to decrease the low end torque for better traction. Larger diameter Collectors will also reduce low end torque. On a good track with lots of bite, extending the primaries or Collectors will pick up bottom end power for a harder launch and awesome e.t.'s. Smaller collector diameters will also increase low end torque.