The fuel leak seal on the Model A fuel line is not the threads, it is the ferule. The ferule is crushed between the nut on the fuel line at each end and the taper where it goes into the valve and the inlet of the carburetor.
What I do is wrap the ferule with Teflon tape, being careful not to get it over the end. The tape will form a seal between the ferule and the seat.
I road test rebuilt Zenith carburetors on my Victoria, which has an indented firewall and a side bowl Zenith. A regular Zenith uses a different shape fuel line so I am constantly switching fuel lines when I road test a Zenith. Each time I take the trouble to remove the old Teflon tape from each end of the fuel line and apply new wraps. With this procedure I am never bothered with a fuel leak at the fuel line connections.
Tom Endy