I'm not sure if it will swap into a 98 GT. It looks to be the right size but the plugs in the back may be different. The Machs also have them, though the redline is different (I think MPH is different also, but maybe not).
Keep in mind that you will have to swap your odometer module which is in the back behind the white plastic. I don't know how fragile they actually are or how hard they are to remove. I just replaced my cluster with another one that had slightly more miles than my car and waited until the odometer matched up. I'm sure a lot of people on this board have swapped the module though. (IMO, that is the right way to do it).
If you don't swap the module, you'll need to disable the PATS (I think 98s have the PATS). If not the car won't start. I'm pretty sure if you just swap the module you should be fine though.
If it fits up, it shouldn't take very long to swap. The hardest part for me was getting the headlight knob off.