This is the site I got this from click. Its got a lot of good imfo I cant stop reading when I go there.

By Doug Anderson
[/align] The Chevy small block was one of the most popular and most successful engines ever designed with over 90 million of them built since the 265 Chevy showed up in 1955. The first generation small block was revised to create the second generation LT1/LT4 that was used for some applications from '92 to '97, but the results didn't satisfy the people at GM Powertrain, so they started all over in '91 and designed a brand new small block. It's officially called the "Gen III" motor, but it's usually known as the "LS1" because that's what it was called when it was originally installed in the '97 Corvette. This new engine family had the same bore spacing as the original small block, but that's the only thing that stayed the same. The Gen III engine was smaller and lighter, it made more horsepower and torque per cubic inch, created fewer emissions and got better fuel mileage than the Chevy 350 it replaced. It was designed to be built in multiple displacements from day one so it could be used in a wide variety of cars and trucks later on. GM also made sure that the overall size and shape of the "package" would fit in a FWD application, too, so it should be no surprise that the Pontiac Grand Prix GXP is available with the 5.3L this year. The Gen III motor that was originally installed in the '97 Corvette as the LS1 was a 345/5.7L version that made 345 hp and 350 ft.lbs. of torque. After two years of successful experience with the 5.7L in the Corvette, Camaro and Firebird, GM had enough confidence in the design to go ahead and replace the first generation 305 and 350 with the new 4.8L/5.3L/6.0L engines in all of their pickup trucks. Today, the Gen III motors are used in the Corvette, the GTO and the FWD Pontiac GXP along with all of the GM trucks and vans. And, the LS1 is winning at the drag strip, just like the original small block did back in the '50s and '60s, so history repeats itself all over again. The GM engineers did their homework when they designed these engines, so they haven't had to make a lot of changes, but there are four engines, each with a different combination of parts, so there are still three rods, four blocks, six cranks and seven heads to keep track of so far. Putting the wrong parts in the wrong engine will cause problems, so it's important to know exactly what goes where unless you want to do it over again. With that in mind, let's take a look at how these engines all fit together. Blocks
[ul][*]
GM has used five different blocks for the Gen III family. There are iron blocks for the 4.8L/5.3L and 6.0L along with aluminum blocks for the 5.3L, 5.7L and 6.0L. [*]
There's only one internal difference in the blocks that rebuilders need to keep in mind. The OD of the cam bores was changed for model year '04, so the later engines require a different set of cam bearings, even though the cam and everything else stayed the same. [*]
All the head bolts have blind holes, so rebuilders need to make sure there's no debris or oil in them before torquing the head bolts down, because the hydraulic pressure will split the block wide open if there's anything left in the hole. [/ul] 1999 - '04 - 4.8L Trucks
The 4.8L block was made of cast iron and carried a 12551358 casting number. This block was used from '99-'04 for all the 4.8L motors. 1999 -'04 - 5.3L Trucks
[ul][*]
The 5.3L engines had a longer stroke than the 4.8L, but they had the same bore so they

 

Good read. Thanks!