The stock manifold is 2.25, but as you know now the greddy exhaust is going to go from the back of your cat not all the way to the manifold.. since its a 01 you have a secondary o2 sensor. If you have to get emissions tested then you pretty much have to find a cat. if you don't then you can get a test pipe or clean out your current cat and use either a 02 simulator or a couple of spark plug defoulers (google) to get past the o2..Both of which is not legal and for off road use only.

But since you cat is already bad you could just bolt the exhaust on and go until you figure out what you are going to do.