Write Van Impe. Or search his site.
He ought to be willing to provide the ref and quote.
Okay..., I just did a Google search of this website, and did it two ways. One way was searching for the name "William Harold". And then the other way was doing a search for the specific phrase "dead sea scrolls" along with the word "rapture".
And in neither one of those two searches did it turn up anything. Now, perhaps he doesn't have any written material on it and it's only in video format. If that's the case, then it wouldn't show up in a Google search (for those words).
The only other thing I can suggest is to write to his ministry and ask specifically for the original source reference for that stated fact in his presentation.
I would like to find it, but it appears to not be used at all -- anywhere on the web.
Is there an email addy on Van Impe’s website?
I can ask Holly to write him, if you think it would take that?