The kit supplied petcock has a male 10mm machine thread and a barb fitting. Using a machine thread (non tapered) will require the use of a gasket and a matching female fitting. Now you're getting too many parts involved. You probably won't have room for everything.
Discard the kit petcock. It's not going to work well with other components. My guess, from the photo of your plastic tank is that the barbed nipple on the bottom will require a 3/8" or a 5/16" fuel line. Auto parts stores will have reducing fittings to help you mate the tank to a 1/4" fuel line. Take the fuel tank with you, explain to the counter man what you're trying to do and if he's knowledgable, he'll provide the correct parts. Tell him you need to adapt a 1/4" I.D (inside diameter) fuel line to your tank.