At the time of the revolution/original colonies slavery was legal, even if not common, in many of the "northern" states. If NS in at the beginning, you have the same N/S split you had very soon after when Vermont joined. even with NS in you'll have enough pro-slavery states leading up to the CW to produce all of the difficulties (tariff issues, internal improvements like canals and RRs, issues with "forcing" slavery on incoming territories where it would not naturally occur {like Kansas}). By the time of the CW the combination of southern senators and some northern democrats was enough to tie the senate in knots, even with the "anti-slavery" shift in the house as the north became way more populous than the south even with the 2/3 clause.
Concessions to the south, like allow the slave trade for 20 years, the 2/3 clause for the house etc. would still be necessary to get the constitution accepted & ratified.
IMHO NS in the USA would cause more butterflies in the War of 1812 than in the lead up to the CW.