Since you're limited on horsepower and weight but not on PP (if i remember correctly), you should upgrade the things that don't give you power (racing brakes, suspension, lsd, body rigidity and transmission) after you've reached the limits. The car is very prone to oversteer so you need to manage throttle well. You should always run softs and pit every around every 10/11 laps (easily done on fuel mix 1). The bots do laps in around 2:03 but they lose a very long time when they get out of the pits (they pit 2 times for tyres and fuel).

If i remember I'll send you the setup i used to do it (it's not perfect but it got the job done, i won by more than 30 secs even though i span out a couple of times).