You can buy a 2 1/4" flange that will bolt to the manifold. You can buy a 2 1/4" to 2 1/2" expansion pipe. Weld the pipe to the flange and now you have an adapter for your 2 1/2" pipe.
The best modification that I did to my old '73 Duster was to run a 2 1/2" exhaust pipe to the rear bumper.
