IMHO the immediate effect of such a thing is the extension of Southern/slave political power for a while longer. Assuming Kansas and another state or two are admitted with initially slave constitutions, you'll probably see pro-slavery senators initially and perhaps some pro-slavery representatives. Given the reality of these states, the economics of slavery etc, it won't be too long before slavery ends in these states, but the southern ability to block things in Congress extends by perhaps 10 years or more. One of the factors in the timing of the ACW was that the ability of the south to basically veto legislation they did not like was going away. Even with the 3/5 clause the free population of the states that would stay in the Union was 4x that of the states that would secede. The northern states would gain a representation (using 3/5 of border states with slaves) of roughly 250,000, the CSA states 2,100,000 - so for representation the "Union" had 22.25 million, the "CSA" 7.6 million. Sure not all of the representatives were abolition minded, but as far as the south calling the tune in the House of Representatives that was gone - and things like public works/infrastructure, tariffs and education (the south would never had agreed to the land grant university program) were important to the south. In the senate, the number of free states exceeded those of the to be CSA, and add in border states the numbers got worse.
All of the above was apparent to the northern politicians, and while there was division over abolition, even those who cared not about slavery were not inclined to give the southerners an extension on their stranglehold on legislation. The border proposed here is pretty ASB, and within 10 years any states admitted as slave would be non-slave.