What is the weight of a 1990 ford F-600?

There's not enough information given. The weight will be dependent on what sort of body it has on it, the length of the frame, any special equipment on the vehicle (such as pumps, water tanks, etc), and so forth. The F600-F750 trucks the company I work for runs generally have a tare weight of between 14,000 - 16,000 lbs, with varying equipment as these are job specific vehicles.