Couple things to think about. I'm sure you have tried to fill the gas tank up. And it is very difficult with the angle gone from the original bed. I fabricated a new fill neck for my flatbed. Runs up the side on the cab, between the bed and the cab. Makes life easier.
Also the fresh air inlet for the EVAP system is incorporated into the fill neck. So you will have to use the original fill neck at least the top half of it. And extend the hoses back down to the EVAP carbon canister that sits under the bed. A big mistake I made was to wheel the truck and ford a few small rivers and creeks. The water entered the fresh air inlet to the EVAP canister and ruined my small EVAP fan and electric solenoid inside the canister. So don't do that before you extend the hoses and gas filler up higher. Having it sit in the new flatbeds bed wheel well cut out wont work. Just to low. I learned the hard way. If the carbon canister gets ruined I.E. the small Fan the whole canister will need replacement parts are not sold separate. Its pricey also. I was mad at my self.
I have more detailed info if you want it. Also some pics that might help. Let me know.