The sum of my experience regarding MK2 flooding centers on two issues.
Issue (1) If you have the Interstate tank, then the force/weight of the fuel on the float needle is greater. This results in the needle requiring more force to seat, so setting the needle seat height becomes more "touchy". It is less forgiving, and must be adjusted more precisely. Adjust by trial and error. I used a wooden dowel and pin punch. Tap the needle seat gently in or out. Keep track of the movements with calipers to find the "sweet spot". If you go too far, you'll suddenly stop/slow down on the highway until the bowl refills. Not far enough and it'll flood.
Issue (2) is like NORBSA says, the newer plastic used on the float seems to be less rigid compared to the older plastic. The problem is with the little tab that holds the needle. If you compare with the old, you'll notice that it is a bit more flexible. The tab bends too much preventing the needle from pressing down hard enough on the seat to create a good seal. This allows the needle to seep fuel past the seal and is especially aggravated if an Interstate tank is involved. Find an old style float on eBay.
My first experience with my new MK2 was the same as yours. I had flooding on start-up, and when the engine was stopped, there were several occasions when a continuous stream of fuel poured onto the pavement from the carb.
Another tip about the MK2, use the rubber flange type manifold rather than the round tube type. I believe Old Britts can supply this. It's a firmer fit that reduces vibration. To further reduce vibration, suspend the carb from a rubber O-ring attached to the frame. Get creative on this one.
Sorry for the bad news. Like you I was hoping for great things when I first installed the MK2. It's a fine carb when it's sorted properly and relieves you of the synch procedure when using two carbs. In addition it's an authentic Brit upgrade.