whats the difference between a short water pump and a long one.

A short water pump fits closer to the timing chain cover than a long one. If you can put your fingers between the back of the pump and the timing cover, you probably have a long pump. Also, with a long pump, the alternator is normally mounted on the passenger side; with a short pump, it is normally mounted on the driver's side.

if i change to a short does the pulleys and belts change?

Yes - acually, pretty much everything changes with pump type.

This is a link to the CRG Pully data - http://www.camaros.org/engine.shtml#PulleySpreadsheet.
You can click on the year of your car and read the original part numbers for the pulleys.
The easiest way to go about this is get all the pulleys and brackets from a car with your type pump set-up if you can.
The belts aren't hard to find for most any chevy application - if you know the years you are working with. Otherwise just wrap a piece of string around the pulleys, tie it off to keep measurement(s) and take that with you to the parts store to get the correct length belt(s).