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That is a 3/8 inverted flare, (at least the one that's visible). Most of the vendors sell one, it's listed as part of the L-88 fuel line kit and may be available as an individual part.
I've got one somewhere. I got the "kit" from one of the vendors and it fit like a saddle on a sow. Just plain ugly. Ripped it off all the way to the fuel pump, made lines from the pump to the filter, and then to the carb using AN-6 fittings (except for the one at the pump). Fits much better. I've made up lines for the 2x4 setup that's setting on the bench waiting for me to get around to installing it.
I see they make a kit and I will go with the inverted flare set up that is pictured....I want the Corvette to appear more stock, rather than full on race look.