Even if you were to add ballast to equal out the weight balance to that of the Boss 302, the simple fact is that the GT500 doesn't grip nearly as much as the Boss. Certain cars in this game have higher grip tolerances than others, not entirely based on weight distribution of the car itself, but programmed into each car individually.
As a drag tuner, most of my time is spent analyzing cars' grip at full throttle. Also, because I use a DS3, it's harder to control throttle input like a wheel. Over time doing this on GT5 and GT6, I have noticed certain cars have substantially higher grip than others - even with identical weight, weight distribution, horsepower, and downforce. This is not just grip tolerance from full throttle on a dead stop, but also grip through-out a corner, grip during braking, etc.
For improved grip on nose-heavy high-horsepower RWD cars, I suggest adding ballast at full rear to get the car to 50:50 or as close as you can without maxing out the weight. You can use weight reduction in collaboration with this to get some cars close to it without making it too heavy to be competitive. If the car is already at 50:50 and still having problems, try adding between 30-50kg over the rear for extra traction. Next, if available, use flat floors to improve grip on corners and reduce low exit-speed wheelspin on throttle. This will slightly hurt the top-end of your car, so be advised it may reduce your straightaway acceleration. Lastly, increase the rear ride height of the car and stiffen rear spring rates, while slightly lowering the front ride height. These are all common methods used to help make the rear of the car bite into the track. If it's still giving you problems, even though it will make you slower, you may have to reduce horsepower with the limiter or take off upgrades.