As far as I know, all 740 steering racks are interchangeable as complete assemblies. There have been at least three manufacturers of racks used over the years---ZF, Cam Gears and TRW. Component parts, such as tie rod ends, are specific to each brand and NOT interchangeable. I don't think there is supposed to be any difference in steering feel between the different types of rack. I suspect there is something wrong with your 91, either something is worn out or the toe setting is incorrect. The type and condition of the tires can make a big difference too.

If there is nothing wrong with the engine in the 91 745, why swap it out? There are plenty of 960s with bad engines available for cheap, and one of those would seem (to me) to be even better to put the diesel in.