Your opening above is a good example of a false argument. You state "...it was dead wrong." That is a false argument because you are stating an opinion as if it were a fact without any evidence or support for your argument. You are also applying your modern view of "morality" to the past without stating facts as to why they were "dead wrong." An appeal to pathos argument regarding slavery is false because you are just confusing the argument with emotion in place of fact.
So exactly what was, according to you, "dead wrong"? Slavery? Slavery has been practiced by humans for millennia in many cultures and societies across the globe. Not just in America. Are you passing judgement on all societies that practiced slavery throughout history? Or just America?
As to the United States Constitution, do you disagree with the idea that a person who was not free (slave) didn't count as a full person in regards to a vote for representation (the 3/5's clause you mentioned)?
Do imprisoned individuals/ felons have similar voting restrictions today?
Should a person who's rights have been legally restricted have full legal privilege?
Was slavery legal during the 18th century?
Are there any legal practices today that could also be considered immoral? What about policies/ laws that allow the taking of property from one person and giving that property to another without equal compensation (redistribution)?
Historically, how did one become a slave legally? Any similarities to today's penal system?
What part of history are you saying was rewritten without regard to facts?