I had such a problem with this, I found a way to extend both the fill pipe, and the overflow pipe by putting in a hollow tube and then another tube down to a half gallon empty pop bottle for each tube.
In a 1/4 mile drive around the block, the main fill tube had overspilled through sloshing and almost filled the water bottle, and the overfill tube had spilled and filled about 1/4 of the water bottle.
After that, I plugged the fill tube (the main culprit) and extended the overfill tube much higher so it would not spill out of it and still release water when I pumped it out of the tank.
