What's the difference between the generation of the BBC's and how do you tell between them?

All I know is a gen 6 does not have a location for a mecanical fuel pump on it. It has to have an electric pump feed it gas.

Carm, try to pick up a copy of "Big Block Chevy Marine Performance" by Dennis Moore. There is plenty of good info there.
Here are a few pages that may help.

pages 3 & 4

Gen IV... tried and true "old school", more or less unchanged since the 396 in '66

Gen V Same Geometry as gen 4, but slightly different ballancing to accomodate one piece rear seal design, started in 1991, early gen V engines had normal provision for mechanical fuel pump block still made in Tonowanda NY, also has oiling differences.
Gen VI, really unchanged from the Gen V, but has a roller lifter cam, was made in Tonowanda untill GM sundowned it for production use, then tooling was moved to Mexico (tolluca?), still made to this day as replacements through gm's service parts division, but not sold to OEM's.

Easy checks:
Gen IV...place for mechanical fuel pump
Gen V....No place for mechanical fuel pump
Gen VI...No place for mechanical fuel pump and non-steel timing chain cover.

storm thanks for the info about the book. true is I have it, maybe I should read it!

gen 4 place for mechanical pump
gen 5 no place for mechanical pump
gen 6 place for mechanical pump and aluminum timing chain cover.

i'm using 1 gen 5 and 1 gen 6 . gen 5 uses mechanical fuel pump mounted on seawater pump and runs off crank via belt. spins twice as fast. as gen 4 mech pump that runs off of cam . gen 5 and 6 use one piece rear main.

also the water pasages are difernt ,, round and kindia triangle shape,, the check the book for inner change for shoure

I just bought that book last week - I spent almost too many hours reading it back and fourth