'The South fought hard to include the "except as punishment for crime" clause in the 13th amendment.'

I don't think that's historically accurate. The 13th amendment was passed in the Senate on April 8, 1864 and HoR on January 31, 1865.

At the time, the senate and congressional seats from the 10 southern states were vacant.

So the only fighting the south was doing was in the civil war, which didn't end until May 26, 1865.

The text itself is identical to the text in the Northwest Ordinance of 1787, which prohibited slavery (but allowed for the return of fugitive slaves) in what would later be Ohio, Indiana, Illinois, Michigan, Wisconsin, and part of Minnesota.