re: Where is the Western boundary of The South?

"The Confederate Constitution of seven state signatories—South Carolina, Mississippi, Florida, Alabama, Georgia, Louisiana and Texas—formed a "permanent federal government" in Montgomery, Alabama, in 1861. Four additional slave-holding states—Virginia, Arkansas, Tennessee and North Carolina—declared their secession and joined the Confederacy following a call by U. S. President Abraham Lincoln for troops from each state to recapture Sumter and other lost federal properties in the South."

Texas has always been a part of the south.