It is

He located his half-section (actually 322.89 acres) in Sections 29 and 30 of Township 1 North, Range 1 East, Willamette Meridian in what came to be known as Willamette Heights, in Portland

it is recorded by:

Certificate No. 1416, Oregon City Land Office. Since it was a half section, he must have been single at the time

An 1852 survey map is available online, and you can identify sections 29 and 30, though it doesn't have the metes and bounds of Neff's section.

This is, today, an area in the West of Portland, much of it still undeveloped or parkland, which was historically some of the most valuable property in the City. Includes the Pittock Mansion, Washington Park, parts of Forest Park, the Rose Garden. The "Willamette Meridian" was established in 1851, a line that runs North from California through Oregon and Washington; it was the reference for surveys in the Northwest. The "Willamette Stone" is placed at the intersection of the Meridian and the East West "base line"; the intersection of these two lines is called "the initial point" and is located on Skyline Drive in Portland. This initial survey work was done by a geologist, Dr. John Evans, who in addition to establishing the Meridian and the base line also accomplished the first geologic survey of the Territory.

Pennoyer was a major force in Portland's history, Neff was perhaps born in 1824 of whom nothing much is heard beyond this litigation; he signs his papers with "his mark", e.g. he likely could not read. There is a record that he may have gone South to California in 1848; many did at the time for the Gold Rush.

Whatever Neff's limitations, his 320 odd acres were well or luckily chosen, there's good reason that the sharp lawyer Mitchell (later a US Senator) and Pennoyer (who was at different times Mayor of Portland and Governor of Oregon) would have been eager to gain them. Before the damming of the Columbia and Willamette, being at elevation was a much better place to be than lower lying property which tended to flood, sometimes disastrously. Neff's section lies some 300 to 500 feet above downtown Portland, where people like the Pittocks wanted to live at the end of the 19th century. Pennoyer himself lived a bit down the hill, his former mansion was on "the Pennoyer Block"; a Target now occupies it . . . this doesn't seem to have been within the Neff section, but without a map it's hard to be sure.

Pennoyer, born in Groton, New York (near Ithaca) and a graduate of Harvard Law School, he is today remembered as a racist-- a supporter of slavery and the Confederacy and known for agitating against Chinese. On other matters he's remembered as a progressive, a supporter of a graduated income tax and business regulation.

The case was first heard by Oregon's first Federal judge, Matthew Deady, a sometime blacksmith's apprentice from Ohio.

Sources:

King, Ralph. “Pennoyer v. Neff: Legal Landmark.” Oregon Historical Quarterly, vol. 73, no. 1, 1972, pp. 60–62.

Miller, George R. “The Great Willamette River Flood of 1861.” Oregon Historical Quarterly, vol. 100, no. 2, 1999, pp. 182–207.

Teiser, Sidney. “A Pioneer Judge of Oregon Matthew P. Deady.” Oregon Historical Quarterly, vol. 44, no. 1, 1943, pp. 61–81.

Lange, Erwin F. “Dr. John Evans, U. S. Geologist to the Oregon and Washington Territories.” Proceedings of the American Philosophical Society, vol. 103, no. 3, 1959, pp. 476–484.

Copies of land maps and certificates can be obtained at the Recorder's Office of Multnomah County; also quite likely the Oregon State Archives, and the Oregon Historical Society has a faint xerox. Title company databases would have these maps as well (paywalled), these maps are still in use in Oregon, though whether they specify the precise metes and bounds of this parcel I don't know. Because this was then a Federally administered Territory, the BLM's online cadastral survey may have it-- it certainly has the maps and surveys.

Judge Deady's papers are at the Oregon Historical Society.

A copy of the 1852 Map of Township 1 North, Range 1 East, is available in a scan online