It's going to cost you a lot more than that for a Hellion kit to make that safely. Remember, the motor in your 99 Cobra WILL NOT live @ 475-500 rwhp. You will be putting in a new motor at that power level. The block is the only thing good in a 99 for that power level. All the supporting mods are not up to par.
With the 04 you can usually find one with a pulley, exhaust and tune thats making in that range and live a long life without even stressing the engine.
Plus, if you dump another $8000 into the 99, it will not be worth the money of the 04. If you ever have to get out of it for some reason, it will be a lot easier to get your money out of the 04 then a modded 99. Just my $.02