I just install my FRPP GTA's last week end and of course faced the same problem, driver's side muffler was getting out one inch more than the passenger's side one !!
Already, starting installation, I found one braket was to be bent to align the tip with the bumper's round part !
After carefully reading all this post, I spent another half an hour under the car and loosen both H pype and Muffler connections, repositioned the muffler with rubber hammer, support it with a piece of wood and fasten all bolts, perfect result !!
Thanks again to all crew of this post for their help !!!