Whats the difference between a long block and short block? Performance and fitting wise? On a big block application?

A long block has heads. A short block does not. Often a long block price includes the oil pump and gaskets. Bolt on your pan and intake and go. A short block may or may not include a cam. Often the oil pump, lifters, timing set, and gasket set are extra. No inherent performance differences.

So it's what someone is selling it with? Strange I never would have guessed! Thanks.