I've got 2 x 3.2's and 1 x 2.6, and I'm sure they all have a 'gasket' between the down pipes and the centre pipes. The down pipes have a welded threaded oval shaped fixed clamp which is usually the issue when trying to remove the bolts as they can shear off in it. Next to this clamp is a tapered metal woven gasket that sits on the end of the pipe on which the flared end of the centre pipe mates to. This woven gasket looks like it is part of the downpipe as it is a tight fit. The two pipes are secured by the two bolts that pass through the loose oval shaped clamp bolting into the fixed clamp on the downpipes.
If you think the bolts are going to shear, you can cut a very thin slot across the clamp through to the bolt, which will release the tension on it and aid its removal. Alternatively some heat will help.
Bear in mind that if you get a 2.5 or 3.0 litre rear box (some suppliers will say that they are cheaper and will fit ok), but the exit pipes are shorter than the 2.6 and 3.2, and will result in exhaust deposits discolouring the bumper above the exhaust pipes, so I would suggest you try to get the correct ones for your car.