Probably need to use a Junction, as I don't think there is a config file for that.
Takes a bit of GoogleFu if you are new to it and a bit confusing sometimes, been many a year since I needed it (GTR2) so I'd be starting from scratch as well.
Yes, Seanus' suggestion is a good one. In a Windows command prompt, you can enter this command to make a link to your new tracks folder, from the original location.
mklink /J "C:\Link To Folder" "C:\Users\Name\Original Folder"
So move the tracks and cars folders to the new drive (say D), then do:
mklink /J "C:\steam\blah\content\tracks" "D:\tracks"
Ditto for cars.
The first path is where you want the new files to be; the second is your current