I'd stay with the 2.5" exhaust unless you are going over 400 HP. Check to make sure the headers don't come with reducers already. My Hedman headers did. If they don't you'll need to order some. I agree with thedartsport above. I have Hedman long tubes going to Thrush welded mufflers and turn downs right before my axle. The pros, it sounds awesome, sets off car alarms, and at night you can see the swirling dust cloud in the vehicle's headlights behind you at intersections. That dust gets all over the rear of your car. I keep a California Duster in the trunk for this reason. It will also resonate pretty bad inside your car depending on your exhaust setup. A turn down setup will cost less and if you decide to go out to the rear later, you can always add to it. I was worried about the exhaust entering the inside of the vehicle as well, so I took a household carbon monoxide detector with a battery backup and threw it on the rear passenger floor and drove around town and left it idling in place for several minutes. The detector records levels of carbon monoxide detected and it showed nothing. I'm sure if your body seals are deteriorated, it may be worse.
