ok, if its from a 97 cobra, it will be a direct bolt-on. if its from a 99+ cobra, it will not fit w/o about $1000 worth of other stuff
before 1999, the cobras had live axles like we do. since 1999, they have switched to the IRS. if it is the solid axle, then all you have to do is unbolt the lower control arms, upper control arms, shocks and driveshaft, then put the new rear end. you wont need to swap driveshafts, since fox bodies, they have been the same length.
they exist, but its not a direct swap. you need the conrtorl arms, the exhaust, the driveshaft, the shocks, the springs and some other things. IRS completly changes around the rear geometry and where everything goes.