According to quote...unquote on Radio 4 today, Clint never used the word 'punk' when he said go ahead, make my day. My memory is that he said it a bit later when he said:

The question you've got to ask yourself is 'Do I feel lucky?' Well, do you, punk?'

Looks like I'm going to have to watch Sudden Impact again. Next thing they'll be telling me there was no red-eyed baby at the end of Rosemary's Baby ;-)