Since you have a catted y-pipe, you can use the rear O2's or not. If you choose to use them, you need to install the O2 sensors, then probably buy more entensions for them. I am not sure if they will be long enough without exensions. You can go ahead and install them, and if the wires arent long enough, then just leave them disconnected. There won't be any problems with them disconnected except for a SES light. Then when you get the extensions install them. If you choose not to run rear O2 sensors, then you will need O2 simmulators or you will need to take the car to a tuner and have the SES codes edited out. I'd say, if there are already O2 bungs behind the cats, then install the rear O2's and buy extensions if you need them. That will be cheaper than O2 simms or a tune.
Oh, and I used 24" extensions for my LT's also, one of each side for thr front O2's