It was only a couple of years that Shelby actually designed and built the engines for Mustang even though after he was gone, Ford continued to use his name on their Mustangs. I don’t think he built the ‘69 Mustang engine. I thought it was more like ‘67-’68.
I’m not sure about that, but from ‘67 onward Shelby Mustangs were built on the regular assembly line instead of being completed by Shelby.