I have a 1981 Ford F350 4x2 truck Dually

The bottom of the box floor is rotted and I was thinking of just getting rid of the whole thing and just making a flat bed.

I found this site that is helpful and a couple of pictures

How I Built a Flat Bed for my old truck

and it seems pretty simple, however looking at my frame, there is a hump where the axle is.

Does anyone have any pictures of their flatbed truck and how they went over the frame hump?

Or any better plans out there for a flat bed

 

The hump is there on all the pickups. You just bolt the bed to the hump, and make up spacers or blocks for the other areas. The official metal beds you can buy come with side cover plates for the frame to make the area where the spacers are look better from the side.

 

 

The fuel situation is a bear. Right now, I have barely enough angle to get fuel in. So I undo my left side bolts and jack up the left piece of the bed and then have plenty of angle. I'm not quite sure how I am going to rectify this issue.

 

I'm going to build a flatbed for my truck at some point, it's more practical for my work than the original bed.

Notice how he ran those channel rails down the chassis, then put his cross-members on top?
I don't know if loading height is a concern for you...... but to keep the deck height as low as possible, I'm putting the cross-members directly onto the chassis mounts - some spacer blocks will be needed because the mounts aren't all on the same level.

It's only 2", but if you're grunting a 120lb toolbox or something up onto the truck, 2" sure helps!

Yep, provision needs to be made for the filler height.