Next, we slid the new filler neck in from the taillight side of the mounting panel so we could leave the cap on it and install the taillight to check for clearance from the back of the taillight to the filler cap. Make sure the filler is on a downward slope toward the gas tank. With that looking good, mark the flange position on the filler neck and take it all apart. Slide the flange on the filler neck to your mark and make sure there is room enough to thread in the 90-degree fitting for the vent tube. Now, we can tack the flange to the filler neck.