Usually the manufacturers will put a relief on the side of the overflow to release any extra. As the car gets warmer and the pressure increases it will force any additional coolant into the overflow. If there is too much it will go out a little hose on the side down to the street. Brand new vehicles may not have the relief on the overflow, but a 98 definitely should..
Finner if you're rad explodes due to my advice I'll drive from Calgary to change it for you
