Depends on whether you wanna do it yourself or take it somewhere. I have a 93 CV and went to midas muffler, the guy said he would just order the factory duals from a Police Interceptor model and pop it on... Dont recall the price but it was rather steep.. For a mercury I would imagine you could go to the parts store and order the H-pipe, mufflers and the rear pipe from the HPP model or the PI would probably fit too. You will also need to get the Drivers side rear pipe and I also believe a muffler hanger...they weld onto the frame.
Snake since you are so close to Idaho there is a place called Barger Matson auto salvage in Nampa Idaho. They have a four interceptors in their yard and a couple grand marcs. Last I remember they may have still had the pipes. I don't know what shipping would cost but it is worth a try. Also I am currently researching hoping up my 4.6 and have started posting my findings in that section.