Mine has a 12 gallon on it. I discovered the manufacturer is out of business, as they were bought out by a competitor. The name escapes me at the moment..
The Company who bought them out still makes a 12 gallon that is with in 1/2" of the same size as the tank I have.. It is also about the same capacity of the original tank in the cab.
I have been considering going back to a fabricated steel tank in the cab. I don't like that much weight out on the fender, as you can see the bow it has caused over the years, from being a bit to heavy. A smaller tank, of course would be lighter when full. 8X12= 96 lbs. of fuel roughly... Quite a bit of weight..
Mine is mounted behind the right hand headlight, in a tight fitting aluminum tray some one made to fit tightly to the tank. It is also in the basket some one built on each side of mine..
https://www.youtube.com/watch?v=Qtd_WXprL4A
Look at the 50 second mark to see mine.
Regards, Kirk