18" wheels

DionR said
Who had the issue? I know Wracks ran 17x9's and had issues, but later on it was discovered he had dropped spindles, too.

An OEM 17x8 Mustang rim has an offset of +30mm, while the OEM 17x9 has an offset of +24mm. This works out to about a 1/4" difference between the 8" and 9" wheel as to where the inside lip sits with the 9" rim being closer. If the 8" rim clears the tie rod, the 9" should as well with a worst case 1/4" spacer.

Sorry, I know this is late, but just saw this had to ask.

The 17x8 doesn't clear the tie rod. The OEM Mustang rims that are 17x8 have a backspace of 5.72", and pretty much everyone I've seen run them has to add an 1/8" spacer to put the backspace at ~5.6" to clear the outer tie rod. A 17x9 with a +24mm offset has a backspace of 6". You'd need to get the backspacing to ~5.6" to get it to clear the tie rod, at which point you wouldn't be able to run 275's because of fender clearance, you'd be maxed out with a 255 and even that would be tight.