Calling @DaveBrt . I think in the states in the Confederacy on the east coast they used standard 4'9" gauge, like the rest of the east coast because much of the original equipment was imported from England.
In what was then the west of the Confederacy 5' gauge was common and the USMRR converted the system to that gauge to eliminate the need to transfer freight.
The gauge of available locomotives and rolling stock determines what you do. It's a d***n sight easier to move rails than it is to regauge locos in particular.