May I know what your application is and how do you plan to use the DAC in your system?
Regarding your output voltage range requirement, do you need the zero scale code and full scale code to be at 0V and 20V respectively? Do you have a target resolution for the voltage output?
You can use the AD5535B as the LSB is 50V/65536= 3.05mV. you just want to be able to maximize the resolution as the minimum output range is 50V.
Other solution that you can consider is using the AD5391 with a gain amplifier (G=4) at the outputs. This solution maximizes the resolution of the DAC and the number of unused channels would be less.