The exhaust restriction is fixed by the outlet size from the manifold to the head pipe. You can make it 6" and it won't matter.
For a mild engine, a single 3" (= to dual 2-1/8") is enough for 200 real HP (rough calculation of area: square root of HP ÷ 24 gives the ID, add .098" for 18 gauge, .118" for 16 gauge to get the OD).
Easiest rule for a muffler to get reasonable noise and best power: the largest one you can fit. The internal volume plays a major part in killing noise and harmonics.