Copied from Super Chevy http://www.superchevy.com/how-to/sucp-1107-ls-engine-swap-tips/
Gen III vs. Gen IV
Despite some significant differences between Gen III and Gen IV cylinder blocks, all LS engines share common traits that include:
* 4.40-inch bore centers (like the original small-block)
* Six-bolt, cross-bolted main bearing caps
* Center main thrust bearing
* 9.24-inch deck height
* Four-bolt-per-cylinder head bolt pattern
* 0.842-inch lifter bores
* Distributorless, coil-near-plug ignition system
The most distinguishing differences between Gen III and Gen IV cylinder blocks are larger bores (on some engines), different camshaft position sensor locations - indicated by a move to the front timing cover area on Gen IV blocks vs the top-rear position on Gen III blocks - and, on most Gen IV blocks, cast-in provisions for GM's Active Fuel Management cylinder deactivation system in the lifter valley.
There is great interchangeability between all LS engines, including between Gen III and Gen IV versions. Cylinder heads, crankshafts, intake manifolds and more can be mixed and matched - but the devil is in the details. Not every head matches every intake manifold and not every crankshaft works with every engine combination. Will Handzel's "How to Build High-Performance Chevy LS1/LS6 V-8s" - P/N 88958786 - is a great reference source that outlines the more specific differences and interchangeability among Gen III-based engines.
LS1/LS6
LS1 5.7L (346-cu-in) engines were produced between the 1997 and 2004 model years in the United States (Corvette, Camaro, Firebird and GTO) and stretching into 2005 in other markets (primarily Australia). The LS6 was introduced in 2001 in the Corvette Z06 and was manufactured through 2005, where it also was found in the first generation of the Cadillac CTS-V. The LS1 and LS6 share a 5.7L displacement, but the LS6 production engine uses a unique block casting with enhanced strength, greater bay-to-bay breathing capability and other minor differences. The heads, intake manifolds and camshaft also are unique LS6 parts.
LS2
In 2005, the LS2 6.0L (364 cu in) engine and the Gen IV design changes debuted. In GM performance vehicles, it was offered in the Corvette, GTO and even the heritage-styled SSR roadster. It is the standard engine in the Pontiac G8 GT. Its larger displacement brought greater power. The LS2 is one of the most adaptable engines, as LS1, LS6, LS3 and L92 cylinder heads work well on it.
LS3/L99
Introduced on the 2008 Corvette, the LS3 brought LS base performance to an unprecedented level: 430 horsepower from 6.2L (376 cu in) - making it the most powerful base Corvette engine in history. The LS3 block not only has larger bores than the LS2, but a strengthened casting to support more powerful 6.2L engines, including the LS9 supercharged engine of the Corvette ZR1. The LS3 is offered in the Pontiac G8 GXP and is also the standard V-8 engine in the new, 2010 Camaro SS. The L99 version is equipped with GM's fuel-saving Active Fuel Management cylinder deactivation system and is standard on 2010 Camaro SS models equipped with an automatic transmission.
LS4
Perhaps the most unique application of the LS engine in a car, the LS4 is a 5.3L version used in the front-wheel-drive Chevrolet Impala SS and Pontiac Grand Prix GXP. The LS4 has an aluminum block and unique, low-profile front-end accessory system, including a "flattened" water pump, to accommodate the transverse mounting position within the Impala and Grand Prix. It is rated at 303 horsepower and 323 lb-ft of torque.
LS7
A legend in its own time. The LS7 is the standard engine in the Corvette Z06 and its 7.0L displacement (427 cubic inches) makes it the largest LS engine offered in a production car. Unlike LS1/LS6, LS2 and LS3 engines, the LS7 uses a Siamese-bore cylinder block design - required for its big, 4.125-inch bores. Competition-proven heads and lightweight components, such as titanium rods and intake valves, make the LS7 a street-tuned racing engine, with 505 horsepower. LS7 engines are built by hand at the GM Performance Build Center in Wixom, Mich.
LS9
The most powerful production engine ever from GM, the LS9 is the 6.2L supercharged and charge-cooled engine of the Corvette ZR1. It is rated at an astonishing 638 horsepower. The LS9 uses the strengthened 6.2L block with stronger, roto-cast cylinder heads and a sixth-generation 2.3L Roots-type supercharger. Like the LS7, it uses a dry-sump oiling system. It is the ultimate production LS engine. It is built by hand at the GM Performance Build Center in Wixom, Mich.
LSA
A detuned version of the LS9, this supercharged 6.2L engine is standard in the 2009 Cadillac CTS-V. It is built with several differences, when compared to the LS9, including hypereutectic pistons vs. the LS9's forged pistons; and a smaller, 1.9L supercharger. The LSA also has a different charge-cooler design on top of the supercharger. Horsepower is rated at 556 in the super-quick Caddy.