I also like your idea of placing a fuel door near the back of the truck. It won't look stock, but it would look clean and professional if done well.
It's likely the best solution. Trying to hide it just seems to be a compromise. It is what it is, do what GM would have done and put a fuel door. However, if you choose to do that, be aware that if you make the door too high, the pipe (or hose) will cut through the box to get to the tank. IIRC, my '85 blazer had a fuel filler 'tunnel' (for lack of a better word) that cut across slightly in the corner where the floor meets the wall. You will likely have to fashion a similar affair in your box. Even the '80's chevy truck may have had the same arrangement ?. Don't mount the filler TOO low or your back to taking forever to fill.
