dual exhaust on a 273

Headers will let the car breath better. More power, torque and mileage. The down side is low hanging pipes and possibly denting them. Also gasket blow out is possible. If that problem comes up, Mr. Gasket Ultrs seals are excellent.
For a quite muffler, I used Thrush turbo mufflers. There not a hi-flowing performance muffler, but the do make a nice rumble when I hit the long pedal. Interior noise is at a min.
On my 400 cid, I went from a stock single to headers and duals @ 2-1/2 with an "H" pipe. 2-1/4 is just fine for the 273.