There's some misinformation in this thread here... and I'm a stickler for details so:
Yes some of the later Fbods got the LS6 block, but there's no real advantage of having it since everything else (except LS6 intake) was still LS1 spec. Ls6's had different heads and cam at the very least. There are only minor differences in the LS1 block vs the LS6 block (see http://www.ls1tech.com/forums/showthread.php?t=213148
). No Fbody ever got LS6 heads.
LS6 heads are 243 castings and have different valves/springs than the 241 castings of the Fbody's --> Earlier Fbods got 853 or 806 casting heads. The LS6 heads had special springs/valves allow them to safely rev higher. Incidentally LS-2's also got the 243 casting heads but with different valves.