Pics of my GMC Paul poly tank. The first pic shows the fill, inside the bed on the D/S wheel well hump. Did not have the expertise to do the side marker fill which seems quite popular and the bed was prepped and primed for paint by this time. The rough texture is my Line-Ex bed liner. Used a fuel filler from an early 90's Chevrolet van. The 1 1/4'' fuel inlet posed a bit of a problem until I tried using a copper plumbing 90 degree elbow. Took it to a muffler shop and had them expand the end where it fits the van filler. Soldered everything together after a test fit to be sure on angle and length. The large fill hose is fuel line hose, that's pricey per foot!. The next smaller hose is also fuel hose for the fill vent line from the filler tube to the tank. The smaller hose is a tank vent, routed and mounted up along the wheel well, used a sintered brass hydraulic fitting for the end. The poly tank had to vent ports, one each side so I put two vent lines. All works well, just get some strange looks when I am at the gas station pumping gas in the bed of the truck.
Hope this helps, if more pics or info needed let me know. I did this almost 8 years ago and had no issues at all with it.