[Big-Foot]

Mike - your rebuilder is correct. The plastic vents were made to be used only on the center-hung float systems. There are new nitrofil (plastic) floats that are far more buoyant than the standard brass floats. If your carb was built using the new plastic floats, the float will ride up much higher and probably contact the whistle. You could use the brass floats and "maybe" get by.
Typically the venting is not a problem unless you are pushing the extremes in acceleration and braking and are having fuel slosh problems (fuel spilling from the vent tubes)..