You're one lucky person to get that many years out of a water pump. At this stage of the game, if you're going to remove the pump to replace the gasket, the common practice would be to go ahead and replace the pump (although the thought of keeping a pump that has not failed in 30 years is somewhat tempting). The procedure is pretty straight forward. The one slightly wierdness about installing a Volvo water pump is that there are two O-rings that seal the top of the pump to the underside of the head, and you have to pry upward on the pump while tightening the bolts in order for those O-rings to seal properly. Even though your car is a B30 engine, you might read the Water Pump section in the 700/900 FAQ's, as the principle is the same.

It's also a good time to flush the cooling system out and consider replacing the thermostadt, hoses, and any other questionable cooling system components .... how's heater control valve? ..... the radiator? ...... is seems to go on and on. (We won't even discuss the heater core.)