I'd consider Mark a Trustworthy Source on this one. Below is the New York Times article, though it doesn't mention his birth name:

Hillstrom Is Cremated; Attacks On Utah Officials
Feature Funeral Of I. W. W. Poet.

Photo: http://www.bloomington.in.us/~mitch/iww/images/JoeHill.gif

FROM: The New York Times (November 26th 1915) ~
Special to The Times

CHICAGO, Ill., Nov. 25

Joseph Hillstrom, alias Joe Hill, poet laureate of the I. W. W., received about such a funeral today as he would have desired, according to his friends.

Anarchists, Nihilists, some Socialists, "bums" and hoboes generally, of whom less than 10 per cent were American, assembled for the ceremony in the West Side Auditorium. Three thousand persons managed to get inside, while outside twice as many battled with the police to get in.

The red flag floated undisturbed at every turn. The pine coffin containing the body of the man executed by the Utah authorities for murder, was draped in a red flag.

There was no touch of religion in the ceremony. Bitter attacks upon the existing social system and threats against the Utah authorities were made by the orators.

The gathering was greatly augmented by the striking garment workers, who chose the occasion for a demonstration, passing contribution boxes and peddling literature.

On a banner that hung over the coffin was this inscription:

IN MEMORIAM, JOE HILL
We never forget
Murdered by the authorities of the State of Utah,
Nov. 19, 1915

The ceremonies were opened by a quartette, clad in overalls, which sang one of Hillstrom's songs. Then a Polish girl, introduced as the "Rebel Girl," by W.D. Haywood, sang a song written by Hillstrom while awaiting execution and dedicated to his sweetheart.

Jim Larkin, the Urban labor leader, and Haywood assailed the officials of Utah. Judge Hilton of Denver warned Utah that a day of reckoning was coming.

Eight men then shouldered the coffin and led a street procession nearly a mile in length. All along the line of march the crowd yelled and sang Hillstrom's songs. Police battled with the disorderly members all the way to the railway station.

At Graceland Cemetery more addresses were made. The body was then cremated and the ashes will be delivered to the I.W.W. for distribution.

I.W.W. Memorial for Hillstrom

Speakers at a memorial meeting yesterday of the I.W.W. for Joseph Hillstrom, poet of the organization, who paid the death penalty for
murder in Utah recently, asserted that Hillstrom had been put to death because the Church, the army, and capital been stung to the quick by the satire of Hillstrom's poems. "If the economic power of the working classes were properly brought home to the capitalist," Frank Evers, a British subject, said, "he would think hard enough to prevent the murder of any 'Joe Hill.'"

Dan