The balance pot should always be 2K but the offset needs to change with the voltage. Last one I built used 1K pot and 1.37K resistors at 400V but 1.5K should be fine at 350V. This resistor has to be 1/2W and higher wattage doesn't hurt as it gets hot.
24V is better so use that.
Those are the correct LED's.
Yup, we forgot to change the zener string values.