We only have 2 o2's in our cars. All parts stores list 4 for some reason. You want the ones before the cats, since thats what we have. They didnt start putting 4 02's in until 96.
The rear O2 Sensors only monitor the efficiency of the catalytic converts (96-up mustangs). I imagine they are very different than the front O2's. I guess you could try them, and see what happens?
I would bet you get a CEL though. www.oxygensensors.com is about the cheapest place I can think of. Unless then sell them brand new on ebay
Scott