The King of Rock-N-Roll wasn't Elvis Presley, It was Buddy Holly...
Gary Busey vs. Buddy Holly.
A little known fact is that Busey did his own singing, and the instrumentals were played by the actors in the flick.
This was Gary's first big screen appearance (1978) although he starred previously in a few lesser known movies - his first was 'Angels Hard As They Come' - 1971.
The man does have some talent as a musician though, and researched the part for the Buddy Holly story vigorously.
Some good background is here:
https://www.imdb.com/title/tt0077280/triviaI could swear Art did an interview or two w/ Gary, also.
Can't find any references though. :-(
Here's the comparison of Gary and Buddy, side by side.
The only thing that made Elvis 'The King' was that he worked his hips like a wh-or-e, and it drove the women mad. That's why the Ed Sullivan show only showed Elvis from the stomach up on his first TV appearances.
(Major eye roll)