Here is my '89 LX with 255/35/18 on 18x9 front and 275/35/18 on 18x10 rear. You have to modify the fender spat and splash shield on 79-90 fenders. Roll both front and rear fenders.
I recently put 17x9 on 255/40s on the front and 17x10 on 275/40s on the rear of my '87 gt will no major problems.
With the front I was running a coil over setup and the wheels were slightly rubbing on the 14" springs so I had to install shorter 12" ones to solve that(won't be a problem if not using coil overs). Now I have no rubbing on the front and that's with about a 1"-1.5" drop.
On the rear I had to remove the quad shocks which aren't needed if you are using boxed upper and lower control arms. I also had to pull the e-brake cables away from the wheels a little. At that point the only rubbing I had on the rear was with the inside lip of the rim touching the wheel well. A couple hits with a hammer fixxed that without any rear noticable dents in the wheel well.
I got lucky and didn't have to roll the fender lips. Not sure if 18" rims will play any role in that. Some cars need it and some don't.