Context: I took the USH SAT II Practice I from the book and I managed to get a 780 on said practice test, is this score representative of what I would get on the actual test?
I can’t really say because the questions in US History will differ with each test. You might have just taken a practice test that gave you questions on all the topics you are well informed about. I’d try taking multiple practice tests and if you do good there, you’ll probably do good on the actual test.