Different engines respond in different ways. The cylinder heads play a HUGE role in overall air flow and velocity. Depending on the cam as well, one set of pipes on one engine may work better/worse than on another. Primary (from the cyl head) pipe diameter has a significant effect on HP, while the collector volume and length can influence where you move the torque curve. I don't think you are going to find the optimal answer, but merely guidance on what works and what doesn't to formulate a ballpark value. And then you'll have to make a decision on the data.
While I enjoy the sound of my 3" Stainless Specialties mufflers, on my particular motor it's arguably too big. If I were to do it again, I might have opted for 2.5" or 2.75" I.D. for a more optimum torque curve. I need less restrictive headers as well (my Edelbrock Victor Jr heads flow pretty good...AFRs would be better).
We all wish we had free access to a good welder and a dyno.
-Dean