...structure will stand for generations to come, just as it did for generations now past...
Is the word now grammatically correct in this usage or even necessary?
Yes, it's correct, and metrically preferable. Say the line with the "now" and without while beating out the rhythm with your hand. Without the "now", you get an extra measure/foot, and you can't match up the rhythms of the two instances of "generations".
|structure will|stand for gener|ations to|come,|just as it |did for gener|ations now|past|
|structure will|stand for gener|ations to|come,|just as it |did for|gener|ations|past|