Just in case you disappear down the same rabbit hole that I did,A few years ago, I had similar problems. Suspecting float issues l cleaned the seats and fuel galleries etc, I set the float heights carefully, assembled and it still overflowed.
Ordered a float needle set & seats with some rubbish off fleabay. Didn't know about this forum then.
I had a spare set of carbs from my own project bike and replaced the floats and needles and it still overflowed! I was getting pretty desperate by now, so I did some research and found a site from the USA (I can't find the link sorry) where a chap, was over hauling his own carbs. He mentioned checking overflow tube in the bowl. Knowing damn well it couldn't possibly be that, I checked it anyway and guess what, it was the overflow tube! I placed the bowl on top of my bench and supported it, poured petrol in it below the level of the tube and petrol poured out of the overflow. I have read that some people suggest that the brass tubes can crack through age and vibration. Someone else suggested it maybe cleaning using an ultrasonic bath, but it's not entirely unknown. Maybe worth a check if you haven't already.
