Based on Thaddeus Stevens' quote from 1867, he supported voting rights for African Americans under the following conditions:
1. If it be just, it should not be denied: Thaddeus Stevens believed that if granting voting rights to African Americans was just and fair, it should not be withheld.
2. If it be necessary, it should be adopted: He also supported African American suffrage if it was deemed necessary for the well-being of the country.
3. If it would punish Southern rebels: Stevens viewed granting voting rights to African Americans as a form of punishment for Southern rebels, indicating that he supported it as a means to penalize those who had engaged in treason.
In summary, Thaddeus Stevens supported African American voting rights if it was just, necessary, and as a way to punish Southern rebels.