My '73 has the stock 20 gallon on one side and an aftermarket tank on the other side...still not sure how much it holds. It is setup with dual electric fuel pumps, I like that idea in case one goes bad...you can still drive home.
I still want to add a big rear tank and use that for daily driving and I'm going to add another fuel pump and a quick connector for a fuel hose, so I can use my truck like a tanker to fill up everything I have!

(mostly my boat, which gets 2mpg)