Ok,
unzip the GT1racemodified.sim file into this directory: Porsche/GameData/Simulation/Cardata.
Then, use the fileditor to load your save file.  On the bottom right, you'll see the the car list.  Check 1998 GT1.
Then highlight it, and click edit.  Now in the sim file, type exactly this:GT1racemodified.sim.  Now close it and
then a window comes up asking you if you want to save, save it.  Now play away.

If you did exactly what I said, yours should work.