Gas tank over flows when I fill up?

When I first got my car, the first thing I did was fill up the tank. I put the nozzle in and set the trigger and left it alone. Just as it cut itself off, I saw gas hitting the ground under the tank. I looked under to see where it came from, and it was just coming out of the overflow tube that exits the car from the bottom of my ground effects on the rear passenger side. Look there, and you'll probably see the same thing. Now when I fill up, I don't top it off and It usually doesn't have that problem.

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Dave & Robin 88 GT, T5
306, polished crank, TRW pistons, underdrive pulleys, cold air induction, 73mm MAF, Xtreme Energy Cam, 3.73's, 1 5/8 unequal length headers, Offroad H-pipe, 2 chamber flowmasters, turn-downs, King Cobra Clutch, Steeda adjustable clutch cable with firewall adjuster