Is 「(i-adjective) + そうだったと思う」 grammatical/natural? The lack of Google results for this kind of construction (relative to the number for the equivalent [i-adjective] + and [i-adjective] + for many i-adjectives) made me antsy about using it. E.g. "I think Jim Morrison seems like he was really interesting." Am I doing something wrong here?

Yes it's natural and it means "I think he seemed very interesting". You are thinking now, and you are thinking about the past situation regarding him.

You can use it when you are recalling your vague memory.

> (I think I thought "He is interesting" in those days, but that could be a lapse in my memory.)

And you can also use it when you are implying the statement is no longer true now.

>