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Someone will be along shortly to offer advice.

I would get in touch with a Ford parts department,and have them look up both the GT 500 mufflers and the stock Mustang GT mufflers,there might be some marks,folds,crimps or numbers that you will know the difference between the GT & GT 500.This is just my thoughts on what I might look into for info,hope this get"s you an answer.Best of luck and keep us posted.

Redlnrz2013 said:

Hello…New to the group and the 5.0 scene.
What’s the best way to find out if mufflers are truly GT500 ? I read about the the 3 horizontal lines and pinched/dents on muffler body…..
Unsure and don’t want to buy stock GT mufflers

Click to expand...

GT500's tips do not have double tubes like the GT ones...

These are my old GT mufflers. Hope this helps. Not real good angles. Thanks.