I hate to tell you Jason, but the header needs to be tuned to the geographical area that you live in, the header style your running, and ecspecially the prop that you are going to run. Ideally, you would re-tune the header for different props. Needless to say, we don't, though normally don't change props radically. My point is, the measurement of my header would not necessarily be the same for yours because of the different areas that we live in and the props we are running, though I am real perticular.
If, after you cut your header you need more back pressure, you can always pinch the stinger some, or change the stinger to a smaller dia., or go with a larger moose can for more expansion.

Regarding the std. vs. long & extra long headers, Joe's right, these are different length pipes from the engine head. The extra long will have less back pressure over the long, and the long has less than the std. Check out Mac Product's website. They show the dimensions there.
Sorry - you're just going to have to tune it yourself!
Kevin