Chevrolet Repair: 86 siburban k10 aftermarket gas tank, gas tank, measurments

Question
I bought this suburban and it has an aftermarket gas tank. the sending unit is also wrong for the tank, as i am on 3/4 at full and run over 200 miles on empty. the measurments of the tank are 31" x 28 3/4" x 12 1/2". i unfortunately don't have a part number. is there anyway i can find out the capacity with out running it completely empty? also i'd like to get the correct sending unit, but how do i do that with out a part #?

Answer
Happy New Year!!

      The tank measurments you gave me total at 47 gallons which is too much for this truck. I'm not sure but I think a 35 gallon is the biggest tank. I would go to a parts store and tell them you have the largest tank and to look up a senging unit for it. This should get you the correct part.

  Have a Great New Year

         Mark