TEST the petcock...
Set tank on bench. Petcock off. Apply vacuum to vacuum port. Should NOT drip even a drop of fuel. If it does, valve side of the petcock is bad. Replace entire petcock.
Vacuum should not bleed down. If it does, vacuum side of petcock is bad. Rebuild using cover set.
Remove vacuum. Turn petcock on or reserve. No fuel should flow. It it does, vacuum side is bad. Rebuild using cover set.
Apply vacuum. Fuel should flow instantly and fast. Pull vacuum line, fuel should stop after a few seconds.
Also check tank vent line for clogs and the presence of the "T" about half way down.
I would also suggest that you have a bad float valve (at least one) if you had gas overflowing the carbs. Fix that too.
Great suggestions. Will check the fuel lines.
How do I "apply vacuum" to the petcock? Don't really want to suck on it

Would a bad float valve cause gas to overflow into the carbs just once? Like I said, it hasn't happened since regardless of the petcock position.