Water Pump Question

I have read warnings not to try to use a non-AC water pump on a 318 motor with AC, and not to use the AC water pump on a 318 without AC. What is the difference. I am removing factory AC from my 1974 Duster. Is it a problem with all the brackets for the AC, or is it a problem with belt alignment? Or something else entirely? Thanks, DJ
Impeller design is the only difference in them. If you search google images you'll find at least 4 different impellers in the same pump castings.