The main bearing has nothing to do with the conrod end float as you say. If the crank has only just been rebuilt then you need to find someone that will do it properly this time around.

The big end thrust washers do tend to wear but by very small increments. I'd say its been rebuild incorrectly. Did you not check it before installing it into the cases after it was rebuilt?
_________________
If I have to take the carbs off once more...