The spring crud has come and gone and I still have that overflowing carb issue. Poking around for a new carb (yes I'm ready to punt), I ran across this tasty little tidbit:
"What caused the overflowing was the float binding on the overflow tube. There is not much clearance between the float and the overflow tube. If any difficulty is experienced removing the float bowl, there is a good chance that the float may be bent towards the tube, and, as a result, binding on the overflow tube. Having set the float level to 19mm, and squaring up the floats, the problem has been resolved. I hope this extra bit of insight is useful."
I've noticed that the float bowl goes on with more difficulty than expected, so it's worth a look.