I'm starting to put the fuel system together for my buggy and am really confused. Should I use NPT fittings or AN fittings? Or both? Is one better than the other? The fuel cell I have is set up for -6 AN fittings, and the fuel rail on the engine is NPT, so I'll need to convert one of them to whichever method I choose. And what size should I be using? I'm feeding a stock V-8.
Plumbing can get confusing very quickly, and it doesn't help that there are so many options out there. We'll handle your last question first. As a rule of thumb, 3/8-inch hose is adequate to feed a stock and even lightly modified V-8. You can get away with 5/16-inch hose as well, but 3/8-inch is a good choice for reasons we will get to in just a moment.
As far as sizing goes, how does 3/8-inch equate to the dash numbers of AN fittings? Well, the dash numbers of AN hose is the diameter of the hose in 1/16-inch increments. For example, a -10 AN hose is 10/16 inches in diameter, in other words 5/8. A -6 hose is equivalent to a 3/8-inch-diameter hose, and this is likely the port size of the fittings on your fuel cell. Although 5/16-inch hose is a common size in automotive applications, there is no AN equivalent, hence why we recommended 3/8-inch hose over 5/16-inch hose.
As for which style of fittings to use, that's really up to you. Both methods are suitable for high-pressure fuel systems when used with the properly rated hose. AN fittings are "racier" than everyday NPT, but they're also more expensive that ordinary brass NPT. Conversion fittings are readily available to convert either your fuel cell or the engine to one style or the other.
Most recently we've been using push-lock fittings, and we really like them. Rated for up to 300 psi, push-lock hose is really easy to use because you simply push the hose onto the fittings. You don't have to build the fittings like you do other styles, and cutting the hose is really easy. Best of all, you avoid the braided stainless steel, which is a pain to deal with and acts like a saw to anything rubber or plastic. You can get push-lock hose fittings in both AN and NPT varieties.