I think a better translation for the second sentence would be “I expected that I ate sushi.” Perhaps.
Let’s look at the context. The expectation is past tense, so that means we don’t expect anything presently. Now, the verb is also in past tense. So the action must have occurred already.
Therefore, I think すし を たべた はず だった means “I expected that I ate sushi,” carrying the context that you already did eat sushi. Thus, your expectation was indeed correct.
On the other hand, すし を たべる はず だった may mean “I expected that I would eat sushi,” in that you did expect to eat sushi, but you didn’t actually eat the sushi.
I’m not sure, maybe someone else can clarify this.