your gas gauge fluctuates depending on how much fuel is in the tank. normal condition.

 

Well, that's normal, but it also is a known problem on the second generation Blazer. It's so well known that GM made the fuel sender a separate part when it was originally part of the fuel pump assembly and replaced only by replacing the fuel pump.

 

98+ Fuel Gauge Diagnosis

 

Your gas gauge is going nuts because you utilized a used pump...
Once the resistor for the sender unit dries out from being pulled out of the tank and exposed to the atmosphere, they almost always act up.
This is a very common problem even with Blazers that are just ran low on gas all of the time.