Us guys with Falcons have a different problem, the bigest we can fit between the gas tank and the leaf spring is a 6" round muffler.
I'm going with a Dynomax Super Turbo Muffler, either WLK-17741 $39.88 2 1/4 in. Inlet 2 1/4 in. Outlet, Case Length 14†overall length 19 ½†Outside Diameter 6†Aluminized Steel, or WLK-17230 $73.88 2 1/2 in. Inlet 2 1/2 in. Outlet, Case Length 16†overall length 21†Outside Diameter 6†Aluminized Steel.
A 2" exhaust pipe will be fine at engine speeds below 4,000 rpm. Above that backpressure will start to hurt performance, use 2 ¼†exhaust pipe to 150 bhp and 2 ½†exhaust pipe for engines producing 150 bhp and above.
Thats for a single outlet, if your running duals 2" is more than adequet.