I've finished updating my research with information from "Clifford Ancestry", compiled by my great uncle Dick. The back of the book contains several pages of notes on the research, and I thought I'd reproduce them here.
"We have been unable to discover any record of their marriage, but the fact of the marriage is roved by a combination of family tradition, a short biography of Jonathon in the History of Plymouth, New Hampshire, and a quit-claim deed in the land records of Barnet, VT (in the Vermont Archives in Montpelier). The quit-claim deed also proves Sarah's parentage.
According to family tradition, some of Jonathon's sons walked to Indiana and entered land in Fayette County; then the parents and younger children joined them. Jonathan and Sarah are buried in Fairview Cemetery in Rush County, just west of the Fayette County line."