IMDB doesn't record any such career overlaps for these actors: Jackson's career is not shown as beginning until 1972, a year after the release of "Dirty Harry" where Eastwood delivers the "punk" line and Dean's short time in Hollywood (1951-55) shows multiple TV appearances with a few movies, none of which appear on Reagan's list of TV shows or films.
It was a JOKE....and a good one!
There was a remarkable episode of G.E. Theater where
James Dean and Ronald Reagan appeared together in a drama called ‘The Dark, Dark Hours’, where Dean played a home intruder and Reagan the father who must protect his family. Here are the highlights from that show broadcast live on CBS Sunday, December 12, 1954.
Dean’s character was threatening the doctor, to have him patch his friend. “That is a .32 caliber bullet. It is very small. I could probably kill you before I would die.
“Do you feel lucky, punk?”
General Electric theatre, 1954.
http://www.cbsnews.com/video/watch/?id=6419376n
http://www.tvparty.com/movdean.html
And I bet you can find it on Youtube where I watched it.