I had that problem on my Camaro when I first got it, the breaker bar and extensions did no good. I finally used an electric impact wrench on it, and that worked. I’d say keep hitting it with your impact. If you get some of the nuts off, put antisieze on the studs and tighten them up again until you get all the nuts loose. Having only one nut on, and all the clamping force on that one last nut only makes it that much harder to remove.