I agree with the above - publishing as an undergrad isn't the "norm." I don't think prolonging graduation is a good idea. If you have a strong GPA, good SOP, LOR, etc. then your application is in good shape. Though, instead of prolonging graduation, you could consider getting an MA in history before the PhD. This could provide more opportunities to publish and present at conferences. I'm just now completing my MA in history have presented at numerous conferences and have published twice. Also, search H-Net announcements for undergraduate journals that are publishing and submit your work to those!