Hi mate, i think you'll need a new gasket to make a seal between the centre section and cat - the old one will be in the same condition as your exaust by the sounds of it!

You'll need some exhaust sealing paste too, making sure you get an air tight seal between the back box>middle section>cat, new clamps and maybe some new rubber hangers depending on the condition of the old ones.

Should be pretty strait forward to get on, its getting the old one off thats a funking barsteward...!

I think the centre section and back box are a complete unit from Ford - replacements are in two sections so if the back box goes, you can saw it off and replace that one bit, rather than everything.

But, if you doing both, you'll only need to disconnect from the cat and then fit the new one (unless the back box runs between the body and the axle - i can't remember).

Good luck