Well, I have a Dynomax cat back and it's all 2 1/4" with a 2 1/2" tip. But here's the problem, the cat has 2 1/4" in and out. I'm going to have to cut the cat off because it is rusted on. And since the pipe that the cat's connected to from the engine is 2 1/2", I went to NAPA and the manager gave me a 2 1/4" ID reducer and I will have a friend weld it on to the cat to ensure a good seal. And since the cat I bought is only 11" long, a lot shorter that the stock one, he gave me a 2 1/4" ID, 2 1/4" OD connector to cut to length. So basically what I'm getting at is: this the right way to go about this? And how long overall should the set up be? (from the beginning of the cat (if it was stock))

Thanks to anyone who would offer advice

hang every thing in place except for the piece of pipe you need to cut to length, then measure the gap, remember to leave enough for things to slip together.