Leaking fuel fittings

Maybe you have the wrong fittings screwed together. Some pipe fittings sizes will "sorta" go into some inverted flare and vice versa

It would pay to "learn you fittings."

The major types in these old girls, and I'm talking what they CAME with meaning U.S, SAE stuff is

pipe thread

inverted flare

SAE flare (45 degree)

Some proprietary fittings AKA power steering and A/C hoses
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Aftermarket you can add AN (flare) which is different than SAE (45 degree). AN has different thread sizes in some cases than SAE, and has a 37 degree flare. In aftermarket automotive, AN is often aluminum or other alloy, or sometimes steel, and in "tractor" / industrial speak is often known as "JIC"

Pipe thread is tapered and "called out" by a given "nominal" ID. IE "1/8 pipe" is roughly 3/8 OD

Inverted flare is used mostly on brake fittings and fuel, and is "called out" by tubing size, but brake tubes often have special non-standard nuts such as in the master cylinder, prop valve, etc, so that they must be assembled (OEM) properly

Fuel line is most often 5/16 with standard 5/16 inverted flare fittings.

Never, if you can avoid it, use "compression" fittings on anything automotive. Certainly NOT brake lines, and vibration is reason enough on anything else.

For thread sizes and other specs, google such things as "inverted flare thread size" etc

A sample

myfui.com/images/Thread%20and%20Port%20Guide.pdf