MK-IV's were prototypes thus would have had to run 3 liter motors in 68.
As Rick said didn't happen and if it had they wouldn't been competitive against 5 liter "Sports Cars".
After 67, homologated cars were allowed up to 5 liter powerplants, hence the 302 in both '68 and '69. Nothing wrong with the 289 mind you but, as it's been said, there's just no substitute for cubic inches...