Learn Spanish (Latin America): Hacía meses que los escombros llenaban la calle. - The rubble had been filling the street for months.

We use Hacía because we are talking about a point in the past. If you use Hace meses que, it means the rubble has been filling the street up until now. Hacía meses que means the rubble had been filling it up until a certain moment in the past.

The English translation says "had been filling", so why does the Spanish use the imperfect tense llenaban instead of a past perfect tense?

When you use the Hacía + [amount of time] + que structure to describe an ongoing action in the past, the main verb is simply conjugated in the imperfect tense (llenaban). The grammatical structure itself already carries the "had been doing" meaning, so a complicated compound tense isn't needed.

Why is los escombros plural when the English translation uses the singular "the rubble"?

In Spanish, the word for rubble or debris, escombros, is almost always used in the plural form. Because it is plural, the verb matching it must also be plural (llenaban).

Can I change the word order and put the time expression at the end of the sentence?

Yes! You can say Los escombros llenaban la calle hacía meses. Notice that when you put the time expression after the main verb, you must drop the word que.