Its a 98. So would a GPS speedo be the best option?If you are running a 4.6 EFI you need the electronic signals running through the computer if the car will not run correctly. It would be easier to change the speedo to electronic. Another thing to think about is what year your gearbox is. 96-98 used a VSS signal and the 99-04 uses a VCC signal. Running a VCC signal through a VSS compatible computer will drive it nuts and visa versa.
So I just need to pull the white plug out and put a cable in?The all digital OSS sending unit was introduced in the Mustang with the 99 model. The 96-98 T-45 uses a VSS like the Fox Mustangs used, but the opening for the speedometer cable has a white plastic plug in it. That plug can be picked out and a Fox type speedometer cable will click right in!