The conditions at lower altitude more favorable for an airship. 65Kft is where the corrosive ozone is at it's peak. At 65K, the ability to cool the electronic equipment is severely reduced because of the air density. Even though the external temperature will be between -50 to -60 degrees Celsius, The ability to transfer heat away from the electronic equipment and motors is reduced using air cooling. At 65K feet, the ability to maneuver the airship will be diminished because of the lower air density.
From the dimensions given on this board of the Sans II. That ship does not have sufficient volume to achieve 65K feet. The HAA prototype will be 500 feet long, 160 feet in diameter, with a volume of 5.2 million cubic feet, i.e. more than 25 times the size of the average Goodyear blimp.
Lets not forget the 90 meter/second wind speed at 10km altitude (about 30kfeet). Getting to 65K is a very difficult task.