Leak seams to be from bottom of tank an not from threaded steel tap. On bottom of tank is threaded plate bonded an riveted to tank. I fear I disturbed bondin when tighten up tap? Hope that explains it better.Was yours the plastic screw tap . They can be replaced. If its metal screw tap and filter I'd try more white tape on it
I used it on @REStracTORATION s recommendation worked a treat on a Massey tank . I cheated a bit and painted round the inside the outlet with a paint brush easy enough on a tank with a fuel gauge hole maybe more difficult on a 35 tank though.You can get stuff you put in the tank and swirl around to seal them, kind of paints the inside. I think I would do that before brazing.
Frost Restoration sell it online