18" GT4 Wheels/Falken Tires

DR_

Lots of potential issues but it probably will work. First off there are lots of years of Mustangs so that doesn't tell much besides the 5x4.5" lugs are probably the same as our 5x114.3mm lugs. However that is only one part of the equation.

The center bore on Mustangs vary between years and also vary front to back in some cases. On aftermarket wheels the center bore are usually pretty big and then different sized rings are used to make the center bore specific to different cars. I don't know if what you are calling the GT4 wheels have specific center bore to certain years Mustangs.

The next potential issue is the offset, which is how far out from the hub the wheels sit. Since this sounds like a square setup (all four tires and wheels are the same size) you would likely need to use spacers in the rear to get it to look good.

Another issue could be that the spoke design of the wheels may not clear your brakes however on the non-sport brakes that isn't usually an issue.

If you get us the offset and center bore we can probably tell you if they will fit.