I don't think the setup of the sentence we're talking about is that important, frankly. What's under debate is whether or not the meat of the phrase is "You've got another thing coming." The first half, "If you think " really has no bearing on anything. The meaning/gist of the second half is what we're concerned with. And the first example I found in that search has exactly the same meaning/gist as what we're talking about. And there are many other examples as time goes on. If you don't agree, oh well. I have made my mind up, anyway. :)

Take a look at the first hit. 1826, iirc. That's close enough for me. :)

Pare it back even further, and the answer seems clear to me.

https://books.google.com/ngrams/graph?content=another+think+coming%2Canother+thing+coming&year_start=1800&year_end=1900&corpus=0&smoothing=3&share=&direct_url=t1%3B%2Canother%20think%20coming%3B%2Cc0%3B.t1%3B%2Canother%20thing%20coming%3B%2Cc0