Conquered the west? Do you mean New Mexico Arizona or West as in Pennsylvania all the way to the Coast? If the latter, I have a hard time seeing it happen. Illinois, Ohio, Indiana, Michigan, Wisconsin, Iowa, Minnesota had no desire to be part of the CSA and would have fought hard. Only way I see the CSA even remotely having a chance is if the U.S.A. splits again dividing the power of the north. Keep in mind the industrial and population power of the north was just ramping up and even then the rebelling states was never able to project power into the North for any sort of extended time.
I get your point, but they seemed to do well enough together in Texas.