I have a 83 blazer, the sending unit in the gas tank needs to be replaced. The local chevy house said they made two different size tanks for this model and they have two different sending units. Is there anyway I can measure the tanks or do each size have a different shape? I havent had the blazer long enough to know how many gallons it holds. Any info would be appreciated.

a 25 gallon tank should measure 29"x28"x11"
a 31 gallon tank should measure 29"x28"x13"

the suberbans ran the same tanks but they also could have a 40 gallon tank

the 40 gallon tank measures35"x29"x13

Thanks, Now I can order the part before pulling the tank!