NEW EXHAUST, HELP ME OUT!

The Force II kit for an 02 GT is #17267. The much preferred American Thunder kit is #17312. I'm not sure where you are getting the description from so I can't really comment on that. It is 2.5 inch that you want.
As you can see from the two pics below, the H pipe at the bottom bolts up to the exhaust manifolds, then at the other end, the cat-back system bolts up to that to complete your exhaust system.

http://www.flowmastermufflers.com/cgi-bin/flowmaster/search.html
Rather than buy cats, you really want a mid pipe, an H pipe is often preferred with (chambered) flowmasters. MRT makes a high flow catted H pipe for your car...here is the link--->http://mustangracing.mrt-direct.com/hpipe.php Best of luck and welcome to StangNet!!!