Please check whether the verbs are in correct form.
Only a few decades ago reputable scientists (had declared) that it (was) impossible to walk on the moon. But in 1969 Neil Armstrong (stepped out) of his space capsule and (made) his famous statement:’ that’s one small step for a man, one giant leap for mankind.’
Am I correct? If wrong please suggest correct form as soon as possible.
Looks good, but why 'had declared'? Why not just 'declared'? It's a fact, right? I'd use 'had declared' if the scientists had changed their minds later on, like this,
At one time, they had declared . . . , but later they changed their minds.
Given the context you provided (i.e., reputable scientists from a few decades ago), it's not apparent that those scientists changed their minds. In fact many of them were probably retired or deceased by the time Armstrong stepped on the moon. The reality is, they were provern wrong.