In the mid-eighties GibAir were using the Viscount for the flights across the med to Africa - except when they needed to get one to the UK for maintenance and then they subbed it for the 737. Confused passengers arriving LGW after a long flight.
On the subject of ceiling, I hitched a ride on an empty BAF freighter BFS/SEN one day and we were over 30,000 ft on the way.