can be done on axel stands .....when Keyser replace mine he manage to remove the bolts from the cats to the y pipe under the car but the rear of the y pipe to the system was a different matter....picture this ,
1 x extra long breaker bar
1 x socket
6 x extension bars
he put all the extension bars together + the socket.....i got under the car and held the socket onto the nut with the extenion bars running the full lenght of the car to beyond the rear bumper.
Push on the break bar and bingo one turn and they where free....looked so comical but worked
hope this helps...good luck