I use to own a 1990 SSP Mustang. I even owned a SSP website for a few years, but shut it down.
The SSP mustang used the normal Mustang 5.0 engine with an oil cooler. There was no such thing as a 5.0 interceptor. That emblem is aftermarket. SSP Mustang never had interceptor badges. Ford didn't start putting Interceptor badges on Crown Vics until 1992.
Ford builds (2) types of vehicles for public safety:
Pursuit Rated
Special Service Package
The special service package is a convenience package. So with the Mustang, it meant things like;
1 key for everything instead of separate door and trunk key.
Reloxated trunk release button inside the car
Certified calibrated speedometer
When you buy an SSP Mustang, you want to make sure it still has the buck tags on the radiator support, the oil cooler, and the 140 or 160 speedometer depending on the year.