All credits goes to @Keith Windsor , I just converted these to GSC2013.
Original thread with screens.
Unpack the zip into GSC2013 install dir, or unpack to some temp folder and move the Gamedata folder manually.
NOTE! This pack will replace original Formula Classic skins. Take a backup first in case you wanna go back to originals (can be found from GSC2013 install dir/GameData/Vehicles/F_Classic).