bluenoser,
I did a relocated tank on my truck. It is a 4400 with a 12' flatbed...big rails sitting on the frame and lots of room...should have been easy. It was a [censored].
My tank top was even with the top of the factory frame rail. This creates a "no drop" situation for the fuel filler.
I ended up putting the fill through the rails on the bed, and even then the drop was marginal.
I even contemplated using the factory fill through the cab and running it out the back and into the tank..too funky!
There are nice hatch covers available to mount in the bed of the truck. They have some made of aluminum with neat fasteners, and they compliment the bed area. If I had to do it again, I would use one of these items. They look nice, they are functional, and they get the job done.
Stuart