Looking at your situation, I have a couple thoughts to add.
1) Try to get clear, bright pictures to post...it makes it much easier to see what's going on. In this case, the obvious is there. Can I assume the bed doesn't have a floor in it? I can't figure how you have room to "peel" those pieces of crossmember up like that with the wood in place.
2) When notching an area like you did, think about the strength your removing...I realize many of us (myself included) don't use our trucks for heavy use everyday like originally designed, however a 2" holesaw hole placed where you need the top to be (below the rolled edge) & then the sides cut up to the hole would leave much more strength, look much nicer, ect. The front side I'd guess could have been quite a bit lower than the back based on the curve of the filler neck. If you want to make that look like a "factory" done install (my liking) would be to then bend a piece of 1/2"x 1/8" flat around the notch & weld it in so the ribs would continue up around the filler tube & continue to the other side giving the appearance that GM installed it & the crossmember was made to fit that tank...takes a little more time, but looks, works as proper as can be.
3) At this point, cutting off the neck & attaching a straight tube upward I'd think would likely be your best plan. I did my fuelcell that way & worked out well. Either get an aftermarket cap sold by streetrod suppliers or my thought...with a little effort & a router, a "factory appearing" wooden door could be installed. You would have to thin the board down on the lid so it wouldn't bind against the floor as you tried to open it, but this could be worked out. Here's a couple pictures of what I did...invision a routered area under the surrounding area to place the pocket & a thin sliced section of the board attached to the lid...a little divot in the rear board to stick your finger in. Just my Two Cents, Lorne
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