The earlier Fox bodies had at least two styles of PS rack.
Here are screen shots from Rock Auto for the 1985 turbo 4 and the 1989 5.0.
It looks like the parts have been consolidated into the same numbers. There might have been a ratio difference between models and still a Ford vs TRW style. But using the same mounting hardware and bushings that came in the 89, it should fit and function if it looks the same.
If the 85 SVO rack is orginal it should be a 2.5 turn lock to lock which would be a 15:1 quick ratio. The rack in your 89 is a 20:1 so the SVO will be a bit quicker. Make sue the connections on the TRW rack are the same as what you have on the 89. Been a long time but I want to say some were different.
Those guys would know for sure. If you do not want the SVO rack offer it up to the guys over on pride as they have SVO's and look for those racks specifically.
If its anything like my 83 rack, you will have to change the bushings out to the later model style bushing.. Completely different style of bushings... A little work, but no big deal, I upgraded my 83 to a 03 rack.....