If you have the serpentine belt, you need the later (92) water pump. The serpentine belt turns the water pump backwards compared to the old (90) "V" belt drive.
You can tell by looking at how the fan is attached to the water pump. If it has one big nut, it is the serpentine belt and the later pump. If the fan is attached by 4 bolts, it is the "V" belt drive and uses the older pump. Your truck uses one system or the other, there was no "mix", it was a complete change over when the serpentine belt was introduced. Gene