Wanting to widen my 2007 Ford Mustang GT Bullit Rims?

I have a 2007 Ford Mustang Grey Alloy GT

Ever since i got it i had always wanted to widen the wheel i just have no clue how i could widen them. The Stock Bullit rims are not wide enough to put wider tires in the rear so i want to widen the bullit rims so that i can fit 345 wide tires in the rear, i am doing this for looks not for performance. How can i do this?

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