Few are, like stated above. Find one if you can, they are good for what you want to do!

 

Man thats a real good question!
As far as external clues, I haven't found anything the would identify the difference between the two.
I have been looking through all my LS1 books and did some searching on the internet for you...the problem is that the 4.8 & 5.3 share the same block and heads from what I can tell.
You may want to call the dealership.... maybe they can tell you if there is a PN on the block somewhere that would identify them.

1999 - '05 - 4.8L Trucks
The 4.8L block was made of cast iron and carried a 12551358 casting number. This block was used from '99-'05 for all the 4.8L motors.

1999 -'05 - 5.3L Trucks
The 5.3L engines have a longer stroke than the 4.8L, but they have the same bore so they shared a common block. All of the '99-'03 engines and most of the '04s came with the 12551358 cast iron block, but there were a few that had an aluminum block.

 

If you want to be different, why not try a 6.0 block with a 4.8 crank/rod assembly (3.27 stroke, 6.27 rod length and same or very close compression height as the 6.0l)? You might get away with the stock rods at a lower boost level, but you'd need the forged equivalents for higher boost.

With the stock 4" bore, this setup will net you 328.7 c.i., high rod/main journal overlap for a more rigid crank, low piston speed, and a high rod ratio (with the piston hanging around TDC a little longer to enhance the boost efforts).

This should lend itself quite well to the twin-turbo setup you're talking about. With well-selected, smaller turbos for a quick spool (ex. Volvo), it would really provide a rush as it screams towards 8k.

 

The 5.3L is a great motor. With 3.73 gears I got 19.7 highway MPG in my 6,000 lb. Avalanche, and that was with the Radix on it. The 5300 really is the best balance between power and efficiency in the GM truck motor lineup.

 

You have a great idea, but I can buy a 5.3L for much less than a 6.0L

 

I Have A Few 5.3 Short Blocks