This stuff is not that hard - but it does get confusing.
Fuel line size requirements are a function of both engine demand and pressure.
A larger fuel line is needed if less pressure is being used.
The line from a fuel tank to the inlet of a fuel pump is under a modest amount of vacuum - no pressure - thus it needs to be rather large in order to provide adequate flow. That's why even stock performance engines needed 3/8 lines, and race stuff needs 1/2" lines or larger/multiple feeds.
The line from the pump to the carb (or injectors) - any kind of pump - is under pressure. Depending on the pressure, the line can be smaller than the inlet side. On a single carb you are usually feeding a pair of needles & seats through .110" diameter orifices. Having humungous fuel lines to the carb on a street car might look cool, but its really kinda silly to be anything larger than 3/8ths.
Pipe threads (pipe threads are also known as NPT) are machined on a taper, and the dimensional description/conversion for them seems to have almost nothing to do with anything the normal guy will read on a ruler. The majority of automotive fuel stuff will have a 1/4 NPT thread, while a few race type items will have a far larger appearing 3/8 NPT thread. High pressure EFI pumps for under 500 HP will get by just fine with a spooky small 1/8 NPT thread on the outlet side - remember about the pressure/flow deal.
Most if not all aluminum casting fuel pumps will require an adapter to connect steel tubing lines. Its pretty much impossible to machine a inverted flare fitting into a casting on a production basis - so they use a female NPT thread. An inverted flare fitting is exactly that - designed with a central cone shape to seal against a flare at the end of a steel tube. The steel line will use a brass fitting with a straight thread, and the tube itself will have a 45 degree double taper. The taper seals against the cone machined into the aforementioned inverted flare fitting.
Your required - and very common - adapter thus ends up having a male pipe thread on one end and a female inverted flare at the other end. You can get a wide variety of combinations to fit most common line sizes to most common pipe threads - and even the factory performance pumps used them on many vehicles as well - not sure about your's in particular. I generally use the Carter M6905 here.
