Granting institutions occasionally provide examples of successful grants on their websites. See, for example, this sample grant from the Harvard's Schlesinger Library: https://www.radcliffe.harvard.edu/schlesinger-library/grants
An excellent overview of how to craft proposals is also here: available https://gsas.harvard.edu/sites/default/files/atoms/files/Scholarly%20Pursuits_CURRENT.pdf. See pg. 48 in particular. It is geared towards fellowships and not archival grants, however, it provides some useful examples of how to summarize your project and state its impact.
If you're interested in viewing other successful proposals, contact the archives.research.program@emory.edu.