Courtesy of Greg Fielden's NASCAR Chronicals book.
"On March 30, 1969, Cale Yarborough won the Atlanta 500 in dominating fashion leading 308 of 500 laps in a successful debut for the Blue Crescent Boss 429cid engine"
Many Ford teams as I remember still ran the 427 FE until late in the season only (or primarily) at short tracks and dirt (yes, dirt!) tracks. But IIRC Ford put their foot down late in '69 and told all teams it was the new Boss 429 or zippo for further factory support.