It's not exactly safe unless the article you got from the expired blog is really valuable. As you already know, there are a lot of private expired article scrapers which scrape articles from waybackmachine so any valuable content is literally TAKEN.
So, the only ones you would find are the low quality ones that nobody uses. If you do find a valuable one, you need to make sure it's unique and that no blog is using it. Either way, manually rewriting the article and adding in few information of your own is the best way to make it safe for your money site + it doesn't require much work.