Hello jrpnde, and welcome!! This is the common way to fuel your plane. Pull the line at the needle valve, (the line from the tank). Insert filler line/probe into the tubing and that is a direct line to the tank. You don't want to mess with the short line from needle valve to carb. Pull the line from the muffler as an overflow indicator. When tank is full it will come out here. Don't leave it on as the muffler will be loaded with raw fuel. NOW... you can do several things with the overflow fuel. I used to carry a 2 oz. fuel tank in my box with fuel tubing and short pieces of brass tubing for line connectors. I would "catch" the good fuel, and when little tank is full, pump it back into the fuel jug. I now use a hand crank pump, and instead of the "catch tank", I attach the pressure line I have removed at the muffler, to a long piece of tubing....and route it RIGHT BACK to that open fitting on the fuel jug. Use 1/8" brass tubes for fuel line connectors. This in essence is a "closed loop system" I pump till I see fuel running through that return line.... turn the crank backward a turn or two, and your done. (I am not sure if the Great Plains fittings will accept plain brass tubing and seal good too). You might check other brands for fit/seal, which is important to keep air out. I broke down and bought a Slimline. It is VERY nice.. but not worth the money or really necessary for the above set up.

PS...... I am going to post a different thread about this.... but I use PINK line for the pressure line on the plane. That way I don't get them mixed up. (Don't ask me how I know) I also use pink for the return line to the jug. You can see the fuel running through it easier.
Also ... when storing/transporting the plane,
pull both lines like you were going to fill up.
Connect the open lines with a short piece of brass tubing. This "seals the tank", to keep air out in case all the fuel didn't pump out.. YOU did pump out the un-used fuel.... right??

...lownslo......Bob