As far as ability of an exhaust system to flow enough air for a given amount of horsepower without causing significant back pressure, this is from one of David Vizard's books:
1. For avoiding significant restriction from back pressure, the pipe should flow at least 2.2CFM per horsepower produced.
2. A straight pipe will flow ~115CFM per square inch of area (using inside diameter of the pipe)
Here is some info on Primary tubing size...
http://victorylibrary.com/mopar/header-tech-c.htm