Hello. Tetsu ^ is right, start from scratch (even from a stock version) is always a radical, yet smart, move when you seem lost on a car's setup.
However, as promissed, I'll leave my setup here for you, hope it will help you, but again; I'm not an "expert" and...this is one tricky car. While it can be fast and you'll never lack torque, it's weight distribution is horrible, making it very VERY understeery, especially on braking or reducing speed. It will always have that big fat V8 nose going ahead. It can be good for leading, but for chasing, especially more oversteering (more angle) cars, it's a handfull, imo.
I do love the looks of it though.
Anyway, see if this helps:
(Built for angle, not for speed!)
Car Make/Model: Pontiac GTO 5.7 Coupe '04
Horsepower: 499 BHP (I'm not found of very high HP, and believe me it will only make the wheels spin forever with no speed added)
Weight: 1418 kg
Tires: Comford HARD
Suspension (Fully-Customisable Suspension)
Ride Height
Front: 77 mm Rear: 92 mm
Spring Rate
Front: 10.50 kg Rear: 8.50 kG
Dampers (Compression)
Front: lvl 5 Rear: lvl 5
Dampers (Extension)
Front: lvl 5 Rear: lvl 5
Anti-Roll Bars
Front: lvl 3 Rear: lvl 3
Camber Angle (-)
Front: 2.8° Rear: 0.5°
Toe Angle
Front: +0.10° Rear: +0.15°
Drivetrain:
Differential Gear: Fully Customisable Mechanical Limited-Slip Differential
Initial Torque: 20
Acceleration Sensitivity: 50
Braking Sensitivity: 30
Clutch & Flywheel: Triple Plate Clutch Kit
Propeller Shaft: Carbon Propeller Shaft
Power
Power Limiter: 100%
Engine Tuning: Standard
Computer: Sports
Exhaust: Sports
Exhaust Manifold: Isometric Exhaust Manifold
Catalytic Converter: Standard
Intake Tuning: Yes
Supercharger: Yes
Nitrous Oxide (N2O): No
Body
Weight Reduction: Stage 3
Bonnet: No
Windows: Reduction
Ballast weight: 60KG
Ballast Position: 50% Back