I just put in new Dynamax mufflers and Roe's elliptical 12" cats on a side pipe car. These cats just fit due to their width.They have to be placed exactly in the correct position, if not, they will be touching either the sill or frame.
In addition, if not positioned correctly the cats could also stop the sill from seating correctly. There is a sweet spot and you will have to find it. If you have any leeway try to set the cats as far back as possible to get it in the wide part of the sill. When reinstalling try to angle them on, the top of the sill angled down.
I also used a 1/4" drive socket with a magnetic base. This allows you to easily start the screws when reinstalling. Start the most forward screw first. Then start the rear most screw next. I also found stainless flat washers to replace the old washers.
One other problem that I found was that the rivets holding the inner heat shield were either loose or missing. This was on a car with 8000 miles. I replaced the existing rivets with common aluminum pop rivets.