Ditto what 6sally6 said.  It will be alot less hassle having 2.25" pipes on the car.

I ran a comparison with 2-1/4 and 2-1/2 and had better lap times with the 2-1/4 from my headers back. I am not running any mufflers, but have side exhaust with both pipes exiting just in front of the right rear wheel. It looks like the 2-1/2 gives a better peak HP number, but the 2-1/4 looks to have more area under the curve. This means the torque is more usable through the mid range where I spend most of my time. I like the sound of the bigger pipes, but I'll go for the lap times. I am looking to get one of those dynamometer units that mounts to the tranny tail shaft so I can get a better comparison on stuff like this.

The only reasion I would consider going with the bigger pipe is if and only if you planned on putting in a bigger engine some time in the future.  For the 289 2.25" is perfect.  If you plan on putting a 351 or somthing bigger in the future you may want to consider the larger pipe.