454 dually header/exhaust question

[THI]

What year is your truck? Trying to figure out why a single exhaust vs dual exhaust.

From a different point of view, a 3" single exhaust flows about the same as a 2-1/4" dual exhaust if the intersection of the two pipes is not sharp. I have a 1979 C3500 Dually with headers and dual exhaust (it came from the factory with dual exhaust and no converter) and it is currently in need of a new system - plan on using a new set of Hedman Elite Heders along with a 2-1/2" dual system, they are available from Flowmaster and Pypes.