To answer to the question: Here the time periods are defined and so you do not need "had been". [If no time period is defined, to indicate order of two events of the past, earlier event needs to be associated with "had been" and later event needs to be associated with simple past.]
Off from the topical question, as I find, the original sentence itself is incorrect.
1. There is no subject for "were reenacting".
2. If the sentence didn't have the part "were reenacting the battle of Gretyysburg", presence of "the weather was perfect" would have been meaningful. But the sentence here is a collection of fragments.
3. There is no clear referent for "it".
Is the first sentence from any GMAT forum and is this sentence treated as correct anywhere (though I doubt so)? If not, IMHO, it is better not to brainstorm on this.