The 2008 GT500 mufflers are the same as those used on the GT with an automatic. There is nothing different about them, if you are using GT mufflers for a manual now then they will have a little softer tone.
I had them on my 2006. Got a good buy (mufflers and over axle pipes for $85) so have seen them and checked part numbers. I hope you got a good price on them.
It wasn't until the 2010 model year that the GT500 got a different system from the GT