As per your cam specs you have a decent size cam with roughly 40 - 44 degrees of overlap. As Duster 340 said he wished they made 2 3/4" tubing more readily available, I think that 2 3/4" pipe in your situation would be perfect. They do make 2 3/4" tubing in 304L but its very expensive. Its used in very very lage displacment NOS Pro Mod engines, or exhaust systems. While the 2 3/4" is out of the question price wise, with that much cam id use 3" tubing. The key to a good header and exhaust system is carring enough exhaust speed to scavenge the engine. But using too small of a tube will create a choke in the system, while too large will not create any exhaust speed, this in turn will slow down acceleration of the engine. This is why youll see a small exhaust work so well at low / mid RPM but it will eventually choke off as its not big enough. I could go on for days but cant due to time. Theres a lot of good articles on my website to help you guys on exhaust theory if you have time to read. While I specialize in high end headers it might be a good read for many of you.
www.perfweldheaders.com