as long as the rad is too the full mark, youre ok. the antifreeze water mix should not evaporate quickly, but over time it will. and the overflow tank is just that, overflow. on my torino there isnt even a tank, just a drain hose! make sure the rad is at the full mark, then add a 50/50 mix of antifreeze to the overflow until its at its mark. for 99 you probably still use the standard green coolant.
bottom line is, your ok