ive always wondered that too, some sites say that they will, but the Cougars wheelbase is 3" longer.... made me think whenever i saw one. i just went to the exhaust shop and had mine done. sure it cost me 450, but going from a single to duals with Flowmasters.. it was the best money i spent on my car. 3 chamber flows cost more, so i could have saved a lil bit if i went with the 2 chamber ones. the car is a daily driver though, so i didnt want it too loud. and i would do it again in a heartbeat. but im thinking of making my own for the Mach 1. side exit 2 chambers... oh yeah.
this pic was a while ago, taken at school ( like everything else i do to the car, it happens there ) before the traction bars, and the headers as well. the exhaust was still pretty clean there too lol. just after i put the new shocks on actually. now they are all grungy.
From all of the aftermarket exhaust catalogs I've seen, the exhaust is the same back to the rear wheels. (same headpipes and mufflers). Since Cougars have a slightly longer wheelbase, the tailpipes go back a bit further on our Cats before they bend up over the axle.
Also, and more importantly, Mustangs have a shorter rear deck so the Mustang tailpipes are make to end there. On my 73s, that would mean that the Mustang tailpipes would end almost 8" from the rear valence panel. Boy would that look hokey! You can add the length you need in to the Mustang tailpipes, but again, that looks like a cob-job.
I'd strongly suggest having them custom bent. That way not only can you make sure they fit right, but that way you get mandrel bent pipes instead of those gawd-awful looking 'wrinkly-bend' stock aftermarket pipes. You can upgrade the size then at the same time. And for those of us that live in the not-so-nice climates of the country, you can make sure to get aluminized or stainless tubing so our new custom exhaust won't rot away in just two years like the cheap aftermarket pipes do. Also, for athstetics, please use a gentle turn down and angle cut the ends of the pipes. I wish I could post a pic of the 'correct' tailpipe treatment because it is kinda hard to describe.
Both of my Cats are going to need new exhaust systems before going back on the road. I'll probably have to trailer them to the local exhaust shop since I really don't want to run down there with open headers on Alpha Cat! Alpha is going to get 2-1/2" pipes from the headers to the muffs (with a balance tube), then full-bore mufflers (that means 2-1/2" inside cores) then 2-1/4" tailpipes to 1/2" past the edge of the valence panel. This system will be 1/4" larger than the stock pipe size. Beta Cat will get the stock dual exhaust system but using turbo-style mufflers for a bit of extra growl.
A word about mufflers here. Back in my day (many, many years ago), there wasn't the selection of mufflers we enjoy today. You had to watch the muffler's construction since many might have had the correct size inlet and outlet size, but they would neck down inside of the muffler causing undue restriction. You really don't want to be running 2-1/2" pipes back to 2" (or less) cored mufflers! Many of the cheaper, generic aftermarket mufflers are still made this way today.