Larger fuel tank capacity

[Crusty69]

Forgive me if this has been covered already but I searched and couldn't find much. Obviously these trucks don't get good mileage in any capacity, but, has anybody swapped in a larger tank for better fuel range so you can make it farther between fill ups? I want to get the tank out of the cab anyway so I'm looking for the largest tank I can fit under the bed.

I saw someone once using a 70s f body tank and they mounted it so it filled behind the license plate which would be a nice setup. Those tanks are 21 gallons which would give me 5 additional gallons. I'll go this route if it's the biggest thing I can find, but I'd love to get something 28-30 gallons if it's possible. Space may be an issue.