With gamepad, to turn all assist off would be stupid. It's a very difficult car to handle in real life and in the game, you need to have a decent steering wheel in order to drive it with no assistance. My advice, crank all the assistance to at least half, or else get yerself a steering wheel.
I used the controller to beat it. With them off. I think half will just slow him too much. Use 4 ABS and 1 TCS that should do the trick. The Ford GT is a very powerful car, so you will have to feather the throttle. Instead of full throttle in the middle of the corner go 20% then transition into full, depending on how much wheel slip.
