I'm thinking after reading this that there is still a possibility of crud from the gas tank causing the needle valve to stick open and leak gas, no matter how well it is polished or it's condition (if rubber tipped). Probably it would be the case if the leak occurred on different carbs, not just one. You will see after you run for a while.
Possibly you addressed this but it is not clear from the postings. I mean by that the condition of the in-tank filter.
It's what I found when I pulled my filter and checked its condition. The PO had creamed the tank yet left the damaged filter in place. It had a hole in it. With Cream removed (that was difficult) and new filter installed, no more overflows going on three years now.
Think about this.....
The fuel cannot get into the fuel line until it goes through that very fine screen inside the gas tank.
Then the fuel has to through a paper filter before it can go into the carbs.
Do you really think that something big enough to actually hold open the float valve is getting in there? Even after going through a paper filter? And it would have to stay there.
(well vented place). Take a float bowl off . Have a rag handy.
Attach a fuel line or if it is still on the bike gently hold up the floats and turn the fuel on. If the fuel is coming out and you are holding the float up then it is leaking right?
Now let the float drop down for a second and let gas wash out the jet then hold the float up (closed again) . If it is still leaking it is not "crud", it is a bad fit between the float needle and its seat.
That is what those metal float were famous for. leaking when worn.
Look for the shiny ring on the needle.
See what I mean?