I did not have to add any pipe when I installed the full Flowmaster kit on my car. Actually installed the kit myself.
I am running different headers though. I am using the Hooker Competitons not tri-y's. I just had to bend the h-pipe to align up with the Hooker reducers, then weld them up. Actually I did have to trim one of the reducers just a little. (less than 1/4 of an inch) Like allcarfan stated, it will depend on your setup.
Let me tell you, it is a sweet sounding system!!!
John