fyi, today I JUST got my combo fully going with the gt40p heads/intake and stock HO cam. I didn't believe it at first, but that stock HO cam holds it's own even with better intake and heads!
your 89 likely has 2.73 gears in it, at the most 3.27's. I believe you can jack her up, turn the rear wheels one revolution and count how many times the driveshaft turns, and that's your ratio. Better yet, the rear end "should" have a tag on it that says what's in it, it'll have #'s and letters, but within the first few digits, you'll see a 2 and 73 or 3 and 27 possibly with some letters between it.
another way of telling, my mustang with the stock gears did about 75-80 mph at about 2000 rpms. If that's where you're sitting, you've got stock gears. with 3.73's I now do about 65mph at 2k.