Re: 91 taurus 3.0 swap to 98 mystique
From what I remember, you are dealing with two different drivetrain platforms.
When ford introduced the DOHC duratech motors in the taurus and the contour (Mystique equivalent) they used a 3.0 DOHC in the Taurus (different block from older OHV 3.0L) and a smaller 2.5L DOHC in the contour. I might add that the 2.5 was practically shoe-horned under the hood on those things. The taurus generally had more space and a greater need for the added torque
I would very much doubt that you could fit it, nor do I think it would bolt up (I believe the old 3.0L OHV were windsor blocks).
Even if you got the parts to fit, the wiring job would be a nightmare. Furthermore, there is no way that the 3.0L OHV would pass emissions for 1998 as they were two very different generations (OBD I vs OBD II).
The old 3.0L were truly workhorses that lasted a long time, but I believe that the Taurus was the smallest vehile they were ever put into. They were later used in the Windstar and Ranger. If you were to get it to bolt up and fit, you would need to get an ECU, and other wiring components from one of those from at least 1996 or newer. That would probably be in addition to several external motor component (mass air sensor, intake, ect.)