To summarize:
Gen I Small Block: The venerable 1955-2002 design, varying in displacement from 262 CID to 400 CID with aftermarket blocks/cranks making it up to 454 CID. The last of the Small Blocks were dubbed "Vortec" for the trucks and were displacing 305 and 350 CID (L30 and L31 RPO Codes). These engines are NOT the L99 and LT1 engines.
Gen II Small Block: L99 and LT1 engines only. These were a variant of the Gen I SB, but incorporated a new improved cyl head port design that has a much improved flow, especially in the low lift range. The LT1/L99 incorporated a unique Reverse Cooling system based on the 1955-1959 Pontiac V8's with additional steam vent tubes. The L99/LT1 heads are NOT interchangable with any other engines. These engines have no conventional distributor and have a dry intake manifold (no coolant in the intake). This was the first sequential port electronic fuel injection Small Block (94-97 only, 92-93 engines were batch fuel injection like the L98 TPI's). It also included a first optical trigger ignition known as the OptiSpark and had a staggering 720x resolution on the camshaft (360 slots, measured on the leading and trailing edge), the crankshaft timing is based on that sensor also. The water pump is also driven off the camshaft so no belt is needed for safe engine operation.
Gen III Small Block: Starting in the early 90's GM realized that a new design was needed. Evaluations of a DOHC automatic Corvette (LT5) and a SUHC automatic Corvette (LT4) setup to have the same power by GM Powertrain and they had several GM Execs drive and evaluate what engine they liked best. Uniamous the LT4 powered Vette won. They were all shocked, none were allowed to see under the hood until the end of the test. GM started design of a new design, conventional underhood cam. This new design intially got released in the 1997 Corvette as RPO LS1. Most people incorrectly call ALL Gen III/IV V8's the LS1, this is incorrect, but most understand what is meant. The LS1 is an all aluminum 5.7L 345 CID engine with a plastic intake similar to the Northstar intake design. Displacements ranged from 4.8L (RPO LR4), 5.3L (RPO LM7), 5.7L (RPO LS1 and LS6) and 6.0L (RPO LQ4 and LQ9). The 4.8L and 5.3L share iron blocks, but have different cranks. The 5.3L can be bored to a 5.7L displacement. The 5.3L, 5.7L and 6.0L share cranks, but the 6.0L's are all iron blocks. The 5.7L are all aluminum, the 5.3L's are iron and aluminum. The 5.7L has never been in the truck line. The Gen III 4.8L/5.3L and 6.0L have never been in the car line. The Gen III V8's ranged from 1997 to 2004.
Gen IV Small Block: The Gen IV was a revised version of the Gen III. Much interchanges. I don't know too many of the details, but the displacements ranged from 4.8L (RPdO LY2)/5.3L (RPO LY5/LC9) and 6.0L (RPO LY6) iron block in the standard truck line, and 6.0L in the GTO and Corvette (RPO LS2), the 6.2L in the Premium trucks (Denali, Escalade, Trailblazer SS and SSR (RPO L92)), and the 7.0L (RPO LS7) in the Corvette Z06. This is not 100% correct, it is best that I know at the moment so feel free to add more detail if you can! :banghead
The block has been windowed like the old Olds V8's (not THAT bad at least!) to improve bay to bay breathing. And the most significant improvement, the DOD or Active Fuel Management features in the lifter valley to shut down 4 of the 8 cyls to improve mpg. GM has seen upwards 10-20% improvment in mpg with this.
Well, that is best I can do for now. I have a good LS1/LS6 book that has much on the trucks too. But I am at a loss for more detailed info on the LS2/LS7. Note not ALL RPO codes listed, there are many.
More info:
http://media.gm.com/us/gm/en/news/press_kits/07gmt900/GMT 900 Powertrain FINAL 9-15-05.htm