If the fitting is on the bottom, I would remove it and get a synthetic or brass fitting (heavier) with thread and the correct size nipple for the line. Optimally, the threads would be slightly larger in diameter than the hole (drill hole larger if necessary). Straight, angled, elbow, etc fitting (your choice). If necessary, I would cut off X-number of threads so that no more than 3/16" protruded into the tank. The reason for this is so you maximize usage of fuel in the tank. Mock it up for fit. When satisfied, I would give a liberal coating to the threads and where the remainder of the fitting contacts the tank. If necessary, you can use a threaded fitting which might have a slightly smaller nipple than the fuel line and then clamp the line to the fitting. Splice an inline filter in there and be done with it. If by some miracle you can access the fitting hole from the inside, a nut would be great.