2 BBl Dual Exhaust
I did a dual exhaust on a 360 in my first magnum (79). At the time, New York state was being squezzed buy the Fed Gov. on doing a upgrade on the testing via a Dyno machine that also reads MPH and tests the pipe at the same time.
So I did a dual exahust of the manifolds with twin cats and a H pipe (After the cats) and turbo mufflers.
It was still a nice improvment. Go for it with twin cats at 2-1/4 pipe size. Mount the cats as far foward as practical to the exhaust. You won't have to get crazy on how close it is.