I need an AN cap. Which one?

I just installed this in my gas tank...

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Its a Mallory pickup im using since the car is carb'd, and didnt want to hassle with hacking the stock pump up. I just realized something. My fuel pump and regulator are a non-return style, so i need a cap to plug the return port. (the one highlighted with the green arrow.) I didnt want to weld it shut or anything because ill probably need it for nitrous later on.

I called mallory (mr gasket) and noone there knows what the hell im talking about.

This is the information off the paper that came with it:

FUEL RETURN TUBE: 0.375" O.D. (0.275" I.D.)
FUEL RETURN INLET FITTING: #6 AN FLARE (S.A.E. 37*)

Does anyone know what the cap im looking for should look like? Is there an O-ring that goes with it? or should i just ask for a #6 AN cap ???

jrichker said:

The 37 degree hydraulic fittings are a direct substitute for AN fittings as long as it isn't used on any type of aircraft. The local hydraulic supply shop should have a #6 flare cap.

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I found a place this weekend that has AN fittings locally, but they said they have a few different kinds of #6 caps. they have standard ones, ones with a swivel in the end, one with a safteywire hole in the end, and ones with o-rings... i dont think i need the safety wire version, as that tank wont have enough pressure to blow it off.. but im not sure about using an o-ring type. anyone use these before?