dual feul tanks

[dieseltech]

I took the drivers side 16 gallon fuel tank out of my pickup the other day with the hopes of installing a 20 gallon in its place but the mounting holes in the frame dont line up with the tank brackets. I can get the front bracket to fit but the rear is off. I have no problem drilling new holes to mount the tank but i was under the impression that both 16 and 20 gallon tanks were options for both sides of the truck.