I was gonna wait for one more answer, but
Power is probably digging through old photo albums right now, hehe.
1978 was the first year to feature the 5.0 badgeing.
They were on the 1978 King Cobra Mustang II. It was also labelled on the cowl hood.
I believe the
5.0 badge on the fender came out in 1979, on both the Mustang, and the Mercury Capri.