Sure not every democrat was like Larry McDonald but there is a reason Reagan was able to have a defacto trifecta in the first two years of both of his terms and that’s because there was like 40-45ish southern conservative democrats who would go along with him on most of his agenda .
It’s the same reason FDR was unable to pass much of his agenda post 1938 and the same is true for Truman and the fair deal and LBJ post 1966. It’s cause conservatives had defacto control of the house from 1938-1946 , 1950-1952, 1954-1958 , 1966-1970 , 1981-83, 1985-87 despite democrats having a majority in all these cases. So the conservative coalition had a majority 22 of those 62 years and republicans had actual majorities for 4 so you could say that liberals had a majority for 36 years while conservatives had for 26 which is still a advantage to liberals but not as profound as 58 vs 4
It certainly wasn’t all southern democrats but there were a good deal that went along with republicans much of the time
As I said, there was a spectrum of views across the Southern Democrats and Vote View backs that up with the DW Nominate data.
Generally speaking you had Southern Democrats fall into three groups
Conservatives/Dixiecrats (whose scores were out of line with the bulk of their party) almost invariably represented urban areas or the black belt where the racial divide created a simultaneous wealth skew (since most down market jobs were held by blacks who couldn't vote prior to the VRA), or historically Republican/competitive areas.
Party Line Democrats (whose scores were in line with most of the Democrats voting wise) typically were more upcountry or down market whites with much less diversity.
Those in between - right on the tin, middling scores from either area.
The combination of the first and some of third group, plus the block voting that Fuzzy Bear has referenced (seen as essential to preserve the South's relevance within the Democratic Party) is the source of the votes for the Boll Weevil Coalitions that you describe.
However, there was massive support for various New Deal programs in many poorer parts of the South, while the more racially polarized and wealthy dominated areas lacked this dynamic. However, programs that overlapped with Civil Rights (Equal Employment Opportunity Commission and similar programs) and anything that could be seen as pro-Communist would naturally unify the South in opposition.