My two cents is your original idea of it being just after the Civil War is spot on for South Texas. While Texas was in the Confederate States there was not many battles fought on its soil. When soldiers returned their homes were not in ruins like much of the south. This was when the big round up of wild longhorn cattle and the great drives north occurred and the area had a sense of wildness still. Native tribes had not been suppressed or driven north of the Red River or south of the Rio Grande. I know I've not been accepted yet but I think despite having to use muzzle loaders your original idea would be more interesting. 

1868 would be a good year