Complex sentence: Mayflower was sold to Dell Publishing in 1963, with Powell and Lionel V. Fennelly becoming joint Managing Directors, with Powell in charge of editorial and promotional aspects of the imprint, and Fennelly appointed to manage its administrative and financial aspects.
Split into simpler sentences: Mayflower was sold to Dell Publishing in 1963, with Powell and Lionel V. Fennelly becoming joint Managing Directors. Powell was in charge of editorial and promotional aspects of the imprint, and Fennelly appointed to manage its administrative and financial aspects.