A few things:
1. It wasn't 1780, but rather 1784, with the Ordinance for Territorial Governance.
2. Slavery isn't going to be kept out of Alabama and Mississippi. What will happen is that a bunch of planters will move into the future states from Georgia and South Carolina and, when 1800 rolls around, there'll be a large calamity as these territories try to enter the Union around them being allowed to keep their slaves.
3. Kentucky and Tennessee, on the other hand, might very well end up as free states.
4. This is going to drastically alter slavery politics in the US. I've been planning to use this as a PoD for a while to end slavery more or less peacefully. The very first change is that worries about keeping slave-state/free-state numbers in balance are out the window. Free states, or states with active manumission statutes, are going to start in the majority right away. Might the US end up getting bigger, without the worry about balance in the Senate?
5. Slavery will continue more or less as it did in the states in which it already existed. However, I like to think of butterflying Eli Whitney's need to move from New England to the South (he was broke and had no job prospects -- I'm sure it's possible to butterfly that with a PoD a decade earlier) so the ATL equivalent of his cotton gin doesn't get invented for a few years. Since tobacco (the main product of Virginia planters at the time) burns out the soil even worse than cotton, plantation economies were 'on the way out' at the time. George Washington himself had taken to planting grain.
If the planters of Virginia, by around 1800 or so, really start to feel the bite of soil exhaustion, and cotton doesn't present itself as a viable alternative yet, they may be amicable to manumission, an issue that arose occasionally in Virginia right through the 1830's. If Virginia gets rid of slavery by 1810, this is going to vastly butterfly the economic development of the Upper South. Prosperous small farmers growing cotton, tobacco, and other cash crops will drive early industrialization and urbanization in the Chesapeake Bay and upper Ohio River Valley, which will have spill-over effects along the entire Mississippi-Missouri-Ohio watershed.
Depending on where the planters end up going after freeing their slaves, the South ITTL will be even more against protectionism because the majority of its population, instead of merely the wealthiest part, will now depend on primary exports to England for sustenance. Since there's no longer the moral flaw of slavery to harp over in the political arena, the Federalist/Whig/Republican platform of economic nationalism may simply never catch on, instead of being delayed until the rise of industrialism outside the northern coastal areas.
Overall, we'd live in a much, much better world today. We may talk about the 1860's as the decade of civil rights instead of the 1960's. Frederich Douglass would fill in the role of Martin Luther King Jr. Without a war of conquest being needed to free blacks in the South, Southern attitudes towards blacks will be vastly more receptive.