You can buy a cheap flange tool from harbor freight ($40.00) and it would be much easier to cut a square hole than a round one ,measure your insert panel allow 3/4 inch inside and flange the truck panel this will ad ridgidity and prevent oil canning, use a 4 foot straight edge to maintain correct horizontal panel alignment ,cut gas filler panel to fit and tack weld and then check alignment and curveture of the body to maintain proper panel conture ,if correct continue tack weld ,grind , use panel adhesive to fill any gaps , grind , finish with normal body filler and prime/paint . if done correctly should look stock when done , poorly and you'll have a big flat spot on your truck bed ... be patient / measure 3 times and cut once ! You'll get it just take your time and do it once.
none of this can be done in a COLD garage 70 degrees and up . With the correct tools and materials a good compressor and attention to alignment this is a 2 hour project not including paint prep .That gas door looks pretty flat make sure it has the same conture as the body before you do anything .