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Everything Danny said... except...
Even with leaking needle valves, I can't see a whole tank of gas flowing out of the carbs. I'd think after you'd lost maybe a quart of gas, pressure in the line would be very low with the engine (and fuel pump) stopped. My needle valves were worn out (Weber 40 IDF's) and I'd get a strong smell of gas around the carbs about 20 minutes after parking the car. But that would dissipate after a few hours, and never enough gas to see an actual drip.
You say this happened parked with the nose up. I guess if the nose were high enough above the carbs and the needle valves were bad, you might end up siphoning all the gas out of the tank, but I've never heard of such a thing happening - ever.
Did this happen in very hot weather? I'm thinking maybe your tank has no vent line (or the vent is plugged) and pressure is building up in the tank as the sun works its magic on a parked car.
Do replace the needle valves (and check the old ones you take out of the carbs for wear), but I think I'd look at the tank vent line, too.
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