Some very quick points:
1. What is your definition of "the South"?
2. Deep South (defined as SC, GA, AL, MS, and LA, though you can also include eastern TX and northern FL if you want) have a history of bloc voting that actually goes back to before the Civil War. Historically, the Upper South has often aligned with the five Deep South states but the voting patterns are much more in line with the rest of the country.
3. Some electoral history:
1896 -first post Reconstruction presidential election where the Republican candidate won any former slave states
1920 -first post Reconstruction presidential election where the Republican candidate won any former Confederate states (TN)
1928 -Al Smith, the Democratic candidate, carries only the five state Deep South bloc, plus Arkansas, Massachusetts, and Rhode Island. He gets beat by Hoover in Texas, Florida, and the Upper South apart from Arkansas
1932-1944 -last presidential elections where the Democratic candidate carried every former Confederate (or former slave) state
1948- last presidential election where the Republican candidate failed to carry a single state in the former CSA. But the five state Deep South bloc, ex-Georgia, goes for Thurmond, not Truman.
1956 -first presidential election where the Republican candidate carries one of the Deep South five (LA). But Stevenson still wins the rest of the Deep South, plus three other states, despite getting beaten by something like 15% nationwide.
1964 -The Republican candidate, Goldwater carries all five Deep South states plus his native state of Arizona. He gets beaten by over 20% nationwide.
1972 -first presidential election in US history where the Democratic candidate fails to carry a single state in the former Confederacy. Nixon sweeps.
1976 -Carter wins every state in the former CSA except Virginia for the Democrats. This is the last election where the Democratic candidate "won" the CSA.
1984-8 Second and third Republican sweeps of the former Confederate states in a presidential election
2000-04 Fourth and fifth Republican sweeps of the former Confederate states in a presidential election
The above timeline carries a mass of admittedly somewhat cherrypicked data, but the point is that the re-alignment in the was well underway before Reagan, at the presidential level.
The Upper South starts detaching itself from the "Solid South" bloc in the 1920s. Since then there has not been all that much difference between how these states voted (AR, TN, NC, VA plus the border states of MO, KY, WV, MD, and DE) and how the rest of the country voted. Texas and Florida break away first in 1928 and then more definitively in the 1950s. Texas develops its own voting pattern, particular to Texas, and with Florida it depends on where the most recent group of arrivals in the state come from and how they have voted.
The five Deep South states try backing some regional candidates, but by 1964 are pretty much in the GoP camp, except for Wallace in 1968 and Carter in 1976. A few times Georgia, and even more rarely Louisiana will break from the bloc. South Carolina backed Nixon over Wallace in 1968 but that is it. I don't think Alabama and Mississippi have backed different presidential candidates in any election in US history.
Carter was actually the only President in US history from the Deep South, and that includes northern Florida and eastern Texas (I don't think you should count the Bushes), and may have been the only major party presidential nominee from that group of states, though I'm not sure about this. There have been lots of presidents from AR-TN-NC-VA. Carter's election was actually as anomalous as Kennedy's and had a similar effect. If you look at state by state popular vote totals, he was also more competitive than most modern Democratic presidential candidates in 1980. But you can't just keep nominating southern pols, though there was something of a push within the party to do just that.