I have the bed off to relocate the gas tank between the frame rails. I found these frame extensions. Are these normal or has a previous owner modified the frame?

 

Looks like it was shortened and is missing a cross member.

 

I know what you mean, ithe extension looks grafted on. I checked my f100 frame and similarly my rear bumper has a mount that extends over the frame and bolts to it. I'm not sure if it's factory. I wonder if yours are what was left after cutting off the bumper. I wish I had a definitive answer for you.

Paul

 

Mine is a short bed. In this picture I have already removed the crossmember. I'm going to box the back of the frame and replace the crossmember at the end to allow for the tank to ride a little higher.

 

Having the tank ride a little higher will throw another wrinkle at you. You will need to use a top mounted fuel filler in bed or put a different fill site on edge of tank to fill from top.