It is indeed 2-1/4" and I suggest you just shorten the stock midpipe to fit your long tubes as I did.
1. Cut the midpipe just above the upstream cats at one end, and below the downstream cats in front of the crossover pipe at the other end.
2. Discard the cut sections with the 4 cats (should be about 21" long).
3. Unbolt the remaining short upstream sections of midpipe from the stock headers and save them.
4. Remove the stock headers and install the long tubes.
5. Bolt the saved upstream offcuts to the header flanges.
6. Bridge the gap between those offcuts and the remaining midpipe either with straight pipe, or add a pair of straight-through mufflers as I did to keep the sound down.
just keep in mind that the 2 1/4 will hurt you in the long run if you plan on a bigger cam and heads not to mention you collector size is probly gunna be 2.5 so that will be a restriction right there. The way i see it buy the right exhaust once and be done. I have a 2.5 inch bbk off road h pipe and a 2.5 spintech catback then a 2.5 to a 2 1/4 reducer off the mufflers to the stock tail pipes and i spent a total of 300 for the whole set up brand spanking new great flow great sound and the stock tail pipes keep my back pressure in check