At the level (difficult in other words) selection screen, hold L1 and R1 and Ace Level should appear. Unless your are an experienced driver, don't bother. You will get caned. I was beaten by a GT40 in a Corvette C5R.
I didnt actually think it was that much harder.... they raced the same to me... i dunno i have used stock cars, never been beaten though..
Oh well
Peace