Didn't mean to imply AC didn't fit check the engine, logically they would have to assure they got the engine bay right. Shelby just dropped in his engine and transmission once it got across the pond.
As for Shelby American taking 65th in the '63 Le Mans, that's how it's logged here along with the 7th place finish for AC Cars -
1963 24 Hours of Le Mans Results and Competitors