"... that you would reply" is correct. It is the "future-in-the-past" relative to the moment of "I didn't think." English Grammar in Use by Raymond Murphy is excellent. A paper copy is affordable and you can find a PDF on the net. See unit 47 for reported speech. It covers the essentials of the past-in-the-past, the present-in-the-past, and the future-in-the-past.