The GT500 has a pretty unique hose array and should be the same throughout the 07-12 run. The inlet on top should be 1.75". The other mustangs will be smaller than that. 1.25" I believe.

 

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Welcome to the site.
Who ever had the car prior to you could have replaced the stock radiator, it's a pretty common mod..

 

It looks like I had the stock radiator with 1.75 upper hose size. Was having trouble finding it but radiator express had it in stock. Cost me $328 with shipping and I got it in a few days. There customer service is the best I've dealt with in a long time.

 

Please click your upper right avatar/account settings/vehicle details and enter your car year. It then shows under your screen name, which helps in conversations here. We cover multiple years of the GT500 on this forum. (Remember to click Save)

Also, we encourage everyone to put their first name in their signature to make this place more friendly (click your upper right avatar to get there)(You’ll need to save this as well) Let us know if you have any forum questions.