difference between ac cab and non ac cab

[69chevy1]

will i'm having a problem with my project i was going to start doing my sheet metal work on my original cab but the problem is that my body guy is saying is not worth repairing the cab i have so the solution is that my brother in law has another cab but the problem is that he thinks that the cab a non ac cab i'm going to be shaving my firewall and going to run a vintage air kit what is the difference between the two if some body could help me it would be helpful also it is a 69-c10