Grand Island Greenway is written up as Site Y9 of the Willamette Valley Birding Trail. The birding trail guide gives driving directions and notes on where to walk once you get there.
But a reminder for anyone thinking about "chasing" this bird:
"Western" Yellow-billed Cuckoo is a listed as "Threatened" under the Endangered Species Act. Thus at the same level as Northern Spotted Owl, Marbled Murrelet, "Streaked" Horned Lark, and the coastal population of Western Snowy Plover.
Grand Island is one of the sites that has been identified by USFWS as potential nesting habitat. Thus use of playback or other prompting should be avoided, unless done as part of an authorized survey protocol. We wouldn't want to jeopardize what might yet become the first nesting record in western Oregon, in recent decades.
Also, before going out to search for the bird, please go to the link that I gave previously, and post a comment there TODAY to advocate for continued protection of this species. It won't take nearly as long as you'd spend driving to get to this site, unless you happen to live in Dayton.