"Was going to" versus "would be" as the past tense of "will"

Do you think that "was going to be" or "would be" could be used as past tense of will?

I thought this book was going to be your choice.

I thought this book would be your choice.

Do these two sentences basically mean the same thing or are they slightly different?

Answer

They're effectively equivalent, just as in the "real-world/present tense" versions...

1: "I think this book is going to be your choice."

2: "I think this book will be your choice."

You could make the same "pseudo-distinction" that #1 focusses more on the "present status" (the book is currently destined to be your choice), and #2 more on the "future state" (that will arise). But I think it's better to just call it a stylistic choice.