You do not have to remove the tank to do this job. You only need one special tool that you can get at a auto parts store. Its a spark plug thread tapp.
Run the tank down to around 1/2 tank.
Remove the license plate bracket by removing 2 #20 torx screws.
Remove the 6 Philip's screws holding the filler Assembly to the plastic filler bracket.
Remove the 3 #20 torx screws holding the plastic filler bracket and remove the bracket.
Remove the 2 hose clamps on the TANK end and remove the filler Assembly with the 2 hoses attached.
Put a piece of duct tape over the larger of the 2 openings in the tank. This will be the fill tube.
Insert the spark plug tapp into the vent tube opening in the tank and screw it down as far as you can by hand. Using a pair of channel lock pliers, screw the tapp in about 5-6 more full turns.
Using the channel locks, grip the part of the spark plug tapp thats sticking out and firmly wiggle and pull until the inner vent tube comes out of the outer vent tube in the tank.
Cut the tube off on the opposite end from where you threaded in the tapp to about 3 inches long.
Reinsert the 3" long piece of tubing into the vent tube until it is flush.
Reinstall everything back in reverse order.
You now have a 20 gallon tank and you did not have to remove the tank to do it.