I have doubts that a 98 would "plug and play" nice with a 96. Why? Because the 98 PATS system is different from the 96-97 model year.
The PATS system of the 97 is very similar to the 96 model year.
If this were me looking to do a swap before any $$ were spent, I would compare the wiring diagrams connector pin outs between the model years in question looking for any differences. But even if the connectors are pined exactly the same, I can't guarantee they would interchange.
But.........
If you go to the salvage yard site car-part.com and perform a search for 96 model year Mustang GT with MPH, then a list of model years covering 96-98 does come up. So that would tend to indicate that the cluster would interchange.
Could also be possible that a 98 cluster would work in a 96 but not the other way around.
Might be a good idea to get it in writing from the salvage yard before buying.