On pipe threads, you usually get a feel for how much torque to apply by hand. If you are not sure, get an experienced person to tighten and then you do it so you get the proper feel. It's not very much torque.
Tapered pipe threads don't normally unscrew from vibration once tightened down which is why they are specified.

All pipe threads require sealant because the pipe thread geometry leaves a tiny gap at thread mating surfaces thru which fuel will travel and eventually leak out. There are fuel resistant sealants available from Spruce or auto sources. I use a blue Hypalon (sp?) type that works well for thin applications on threads and gaskets.