88 or 89 was the last year for the standard 13 gallon tank. 20 was optional then. 20's became standard after that.
All they did to lower the volume was to extend the filler tube down into the tank. I found this out after purchasing a new 20 gallon tank (that I promptly filled with 25 gallons of gas).
If you have a 13 and want a 20, most auto parts stores can get them fairly cheap. I hate getting these from junkyards just due to rust issues... Since the tanks are the same size, everything from your old tank, including the mounting straps will fit on the new one. Although I usually change the fuel pump at that point...
Of coarse you could just modify the filler to not go in so deep... Wish Id got pictures of the filler tube now.