Short and long are just that. there is a long version and a short version. Most short versions are found on pre-'70's small blocks. All depends on the brackets you plan on using and as long as you don't have a newer vortec block with reverse flow system or 4 outlets in the block. Most cases you have a long pump. Either short or long will bolt up and function the same way.Parts supply house should be able to help you further on your selecton if you show them what you are working with.