This letter shows Smith declining to proceed with a quote on the 1970 cars.
This ended the program.
This is after two years of not being paid ( including storage cited above) in a timely manner, being blamed for all quality issues, it was AOSmith that taped out.
The program end had nothing to do with Shelby convincing Ford. Remember these cars were expensive and not selling well.
1971 models had already been planned.