Yes, it probably is. The V6 exhaust uses 2-1/4-inch pipe, and the Shelby is 2-3/4 inch. This means they'd have to make some mods to your existing pipe (at the muffler end) to make the GT500 mufflers fit.

Another solution is to replace the pipe from the clamps back with Shelby pipes (again, modification to your existing system will be required to mate the V6 pipes to the GT/Shelby pipes), and then the mufflers become a simple bolt-up.

 

Thank you, J.S. The solution seems simple IF you have access to a tubing expander, which I myself don't have... but I expect most any muffler shop would possess.