Well, if you choose an American history spine that is really appealing to you and it only goes so far, you can always just continue on using the spine you’ve chosen for world history if it goes up to 2000 or close to it. For example, Susan Wise Bauer’s Story of the World (Vol. 4) takes you that far and it includes the major points of American history.
And, if your children are young enough (my youngest two are 10 and 12), they’ll be covering module 6 again in middle or high school….and then we’ll have SCM’s upcoming spines to use for Modern Times! (To paraphrase an old song from the early 1980’s, Celebrate Modern Times, come on! Let’s celebrate….) 
As my mom used to say to me, “You’re getting silly, go to bed!”