Water Pump Question....

I have a 1970 Challenger. Built 383. I have cracked 2 long neck water pumps with RPMS up to 7600. If you are not going over 5000 and have A/C, the long neck is ok. If you plan on running hard, I recommend the short thick shaft water pump.

That's sorta my question. I had no clue there were two different pumps. How do I tell the difference?