everyone says to get a 01 or later 6.0 but why? are the blocks/internals different?

how would a 6.0 run with 5.3 heads, both before and after tuning.

 

different crank. the older ones have a crank that is about 1/2" longer and won't bolt up to the 4l60. unless you are don't mind changing it.

 

He is correect about the crank.There is also a difference with the heads.The newer 6.0L have the aluminum heads with the same design as the ls6 heads but with bigger combustion chamber.

2001-2003 (LQ4) part number 12562317
2002-2003 (LQ9) part number 12572035

Both have 71.06cc combustion chamber volume , 210cc intake port volume and 75cc exhaust port volume.