Do yourself a favor and don't even try overclocking a 603. They were never designed to do that. Most, if not all, 603 motherboards have the clock speed of the processor locked into the design of the motherboard. There are no jumpers or contact points where you can make a change.
Do try to find a G3 upgrade card that works in that unit rather than trying to make the 603 do what it was never supposed to do.