A short neck water pump has the gap behind it and the long does not? is this correct? so stock 82 TA 305 had?? any help is appreciated

 

If by gap you mean a space between the block and the pump, then they both have a gap. A long pump has its pulley mounting surface farther from the block than a short one, ie. the long pump is long and the short pump is short.

Stock, all thirdgens use long pumps. Cars with V belts use standard roation and cars with serpentine belts use reverse rotation.

 

so it has nothing to do with how long the neck is. so how do you know when to use a short or a long? or what is the reasoning for 2 pumps roughly 1 inch in depth apart?

 

It has everything to do with how long the neck is.

You use a long water pump if you're using a long-waterpump-style accessory mounting system, and a short water pump if you're using a short-waterpump-style mounting. The short water pump was used up to 1968 in passenger cars (except the Corvette, which used it longer), and the long water pump was used exclusively afterwards.