1" Wheel adapter/spacer

I would imagine the vendors will have the spacers necessary to convert Panteras to GT5 spec.

"But" keep in mind, the Pantera wheel is the same bolt pattern as the Mustang. (5 x 4.5")

cowboy from hell

Ofcourse it is your choice but personally I think an adapter should only be used if you have a death threat.
A spacer is safer but 1" is way out there. The spacer also won't let your wheels sit on the hubs to center them properly.
It has a little bit of risk to it.

I had a 1/2inch spaced wheel come loose at 70mph. The cop said he thought I was dead for sure when he saw it happen.

And people wonder why I'm crazy?

Go to an ebay store named "Accesorries Plus". They have a wide range of adapters/spacers. Some are "hub centric". I think they can make you whatever you need if they don't have it. Of course the best choice is different wheels with appropriate offset, then longer studs with spacers, then adapters.