You might be careful going 17x9 at all corners on a 1990. Most people would say go with a 17x8 in the front. Remember, 1990 is still before Mustangs came with wider inner fenders to accomodate larger rims/tires. Go to
afs. They do a good job explaining stuff on the early Stangs. I would have boguht their kit with the two 17x8's and two 17x9 wheels, but they were on backorder and couldn't give me an estimate on then they would be ready to ship.
That being said....I bought 17x9 wheels for all four corners on my 1990 Stang
! The website I sent you to said they add 18 mm of offset to their wheels, so they should fit. We will find out soon enough. I got my new wheels/tires mounted and balanced, but they are not on the car yet. I have to get a different set of lug nuts to make them fit, which I should pick up tomorrow. So, if all goes well, I can let everyone know by tomorrow night what the fit is like on my car.