Originally Posted by irishguy28

Book an economy ticket; when you get to the seat selection stage, select an Economy Comfort seat (for an additional fee)

As others have noted, do not make the mistake of thinking that KLM Economy Comfort is a separate class/cabin; it is merely a few rows at the front of the Economy Cabin having slightly greater pitch (it's just a "preferred" type of economy seat - like the exit rows, for example - for which they charge extra if you decide you want to sit in one) It is NOT the equivalent of Delta's Comfort+.

Perhaps you should select a Delta flight and book in Delta Comfort+