Remember that in 89 their were quite a few differences between the HO 5.0 that would have been in Mustangs and the non-HO 5.0 EFI engines of other vehicles. Some will be roller, some will be flat tappet, some could have a 28 oz imbalance and not the 50 oz, as well as different water pump rotations and distributor gears. The water pump rotation also affects which timing cover you use... There are probably others I am forgetting. And that is all before we even talk about which engine your Stang came with.