I notice you can install a rFactor Lite version for this which is a little smaller in size.
I tried it with the Historic GT Mod, the Lite version is fine, but damned if I can get the mod to work though. I follow the instructions in the manual, but when I run it, I get GMotor errors (can't open mas files for cars) and the PC crashes. I've tried a few times now.
Not game to install it to my main rF for fear it will screw that one up too.