How would the makeup of Congress been affected had slaves been 5 fifths of a person?

It would have given the Southern states a lot more representatives, and more power. Obviously. Which is exactly what they wanted, and what they proposed. The Northern states didn't want to count them at all because they had no voting rights. It also would have affected their taxes at the time (more people, more tax owed to the feds). Madison ended up proposing the 3/5 compromise that both sides could live with, and detailed his reasoning in Federalists 54 and 55.

(My memory was a little fuzzy on this, so I did look some of this up.)

It is interesting that Madison opposed the African slave trade throughout his life, yet was able to propose that compromise. Today's politicians aren't able to compromise on much of anything, and I would argue that Madison's compromise was far more consequential than (for example) raising taxes on the wealthy.