To the best of my knowledge, the only Cleavland engine in 69 was the Boss 302, and growing up in the sixties, I don't ever remember seeing or reading about the 351C in anything. The 351C started in '70.
You can do anything you want to......ONCE!
aka "my 66 coupe"
There have been documented cases (original paperwork) wherein a 69 produced very late in the model year (June/July) came through with a Cleveland, as some plants were running low on the Windsors. I have seen two in the last 20 years at shows that were properly documented. But as a rule, no, the Cleveland did not make its official debut until model year 1970. And as far as calling a Boss 302 a Cleveland - shame on you!! Everyone knows the Boss is neither a Cleveland nor a Windsor but simply a BOSS! *LOL*