The WA Archives:
http://www.digitalarchives.wa.gov/Home
has the marriage certificate of the marriage on 14 April 1956 at the St Alphonsus Church, Dennis, 25, born in Seattle; Cleo Mae Cooksey, 26, born in Jordan, Montana. Witnesses were Todd Reeder and Doris Cooksey.
--If "your" Dennis was born in 1938, this is probably the wrong person. This one would have been born in 1930/1931.

A family tree owned by "cjjglass" shows Cleo born 5 June 1929, died 6 Oct 2005 in Seattle, married to "Private" 14 April 1956. Shown as having four daughters and two sons. I sent a message to the owner with a link to this thread for possible help.

Ron Bestrom
Tacoma, WA