Have you checked the "Edson Family History and Genealogy"? Vol.1, p. 50 has Ephraim's parents as:
BETHIA EDSON, youngest child of Deacon Samuel Edson (#1) and Susannah Orcutt, born on March 02, 1654 in the Duxbury Plantation, Plymouth Colony, married, in Bridgewater, on Dec. 17, 1676, Ezra Dean, son of Walter Dean and Eleanor Strong, born in Taunton about 1655. They lived in Taunton, Mass. He died between Oct 28, 1727 and Feb. 15, 1732
i. BETHIAH DEAN, b. October 14, 1677, d. November 27, 1679.
ii. EZRA DEAN, b. October 14, 1679(?); d. Taunton, MA July 01, 1737; m. (1) ABIGAIL LEONARD; m. (2) ABIGAIL BRETNALL.
iii. SAMUEL DEAN, b. April 11, 1681; d. February 16, 1683.
iv. SETH DEAN, b. June 03, 1683.
v. MARGARET DEAN. b.____, who married a Shaw
vi. EPHRAIM DEAN, b. ___; m. MARY ALLEN of Rehoboth
I have more on the Dean and Edson lines if you're interested