Likelyhood is the shaft probably won't come apart even if you wanted it to on an 89. I wouldn't be afraid to loose the bolt. Watch it for a while, it should be easy to see if it moves. I would think a set screw should be enough to keep it from coming apart if it does move. Those pieces have pretty tight clearance.
That said, can you shift the motor towards the right a little to gain the clearance you need? A 1/2" motor shift to the right (slot the motor mount plates?) would make a huge difference on the steering shaft clearance.
Gene