Wouldn't you go ahead and source your door and your tank, then given the rubber connector (flex) hose between the two, decide the best fit on your truck? Sorry if I am missing something, but I am thinking it will be where ever it needs to be depending on your aftermarket tank and where it comes out in your fender, plus the size of the door.
If I had a donor vehicle I'd cut the area around the actual door and what I cut out of the original fender would be larger than just the door. I'd MIG in the door and surround after cutting to fit.
But again, I may be missing something. Good luck.
