88GT vert 17x9's SMALLEST TIRE UP FRONT??

I just but 17x9 on my 93 gt and i went with 245/40 up front. The car isnt lowered yet but i have plenty of room to lower it. Have a little rub with the wheel cut all the way.

i know he needs to really chekc if those wheels will work.i put bullits on my triple white 90 vert using that conversion and had to pu 3/8 spacers on front because the roun hole where the center cap goes is smaller than the dust cap on the lincoln rotor.the spacer didnt make them stick out they actually look perfectly flush.but ther is several factory ford wheels that have that issue.

I but anniversary wheels on my car and had to put 5/16 wheel spacers up front cause the rim wouldnt set on the rotor cause of the center of the wheel but cant get my center cap on yet.

235 is a safe way to go, however, one company's 245 may be a different size that another's 245. There are minor discrepancies as to where the actual tread begins, moreover, some companies make their tires more square.

For example, I used to work for Firestone, and being the new guy, I chucked tires every week, so I got pretty familiar with the sizes, and what they looked like. I have a set of Firestone Firehawk 275/40/17, meaning from bead to bead, the tire is 275mm, the sidewall is 40% of that. So, in theory, the footprint of the treadblocks should measure 165mm. I put that tire next to a Potenza S-03 which measured 245/40/17 and it was about 5mm narrower than a 275 (should have a 147mm footprint).
Theoretically, there should've been an 18mm difference in footprint, but Firestone measured the treadblock up the sidewall a ways.

Basically, what I'm saying is that one company's 245 may be wider, so stay away from the brand that rubbed, you can find one that doesn't. or you could just get a 235. lol

I use to tell people all the time to just run 7/8ths spacers up front to clear the hub on conventional rotors and allow the use of the centercap on the rim. You will need to change to longer studs but its an easy task. It works fine and runs the rim flush with the body for a clean look.
As for tire size, dont go smaller than 245, in fact even that to me looks too small and goofy.
The best combo for a Mustang that also does not affect the speedo is 17X8 up front, 17X9 out back with 235 or 245/45/17s up front and 275/40/17s out back. This I've found looks and works the best if you want a wide look. I wanted to do 10.5s in the back with 315s but personally I've found it looks like crap so I didnt do it. The 10.5s stick out way too much!
If you want a better look and you have the extra money, get 18s. The only problem with 18s is the cost of the tires. The rims are just as cheap as 17s.
If he rubs with 245s then go down to a 235 with an 8 inch rim or try the 7/8ths spacers.