The ACTUAL I.D. of AN fittings are:
-4AN = .183"
-6AN = .279
-8AN = .389
-10AN=.481"
So, -6AN is NOT 3/8", though most consider it to be 3/8" and -8AN to be 1/2". -10AN is closer to 1/2" than -8AN is. This it measuring the I.D of the actual fittings. That is the restriction in the whole system.
The stock fuel line fittings (Saginaw) are .257" at each 3/8" connection, or only 1/4". So, stepping up to a -6AN feed line for the fuel system is a big gain. Doesn't seem like much, BUT, there are 750HP sprint cars running on methanol with a single -6AN feed line to the injectors. (methanol takes TWICE as much volume as gasoline to make the same power). Hope this helps.