Go custom-bent locally. You'll be glad you did. Much better than any one-size-fits-all compromise system.
One of the local shops here does custom pipe bending, but he will NOT supply the mufflers any more - too many people were coming back saying 'I don't like the way it sounds'. This way, the mufflers and their sound are the customer responsibility.
I had the custom system bent out of aluminized pipe for my '89 Crown Vic's 393W (Fox chassis 351W swap shorty headers, 2-1/2" 5.0 Mustang H-pipe) 2-1/2" intermediate pipe to my supplied Fox chassis Mustang 2-1/2" core DynoMax Super Turbo mufflers (have the correct hangar brackets on the back for my car), and custom-fit 2-1/4" copies of the stock Crown Vic tailpipes. I made back-to-back passes at the track with open headers and through the exhaust - no change in either ET or MPH. I guess that means the full exhaust isn't holding me back any. Oh, and when the catalysts decided to eat themselves, the pieces-parts went all the way through the system - no remnants were found in the mufflers! Now that's full-flow!, LOL!!!
To keep this Cougar-related, my buddy Terry whipped up the system for AlphaCat from the headers back. I bought the 3"pipe and 3" mandrel-bent elbows and he did the cutting/welding. Tailpipes will be 2-1/2" copies of the stock pipes, bent by the guy that did my Vic's system...