“LS6” intake manifold and cam change were the only substantial differences. Casting #'s are different on the heads but the actual design is identical (just like 799 and 243 heads are the same). The 01+ LS1 uses the LQ4 cam which is actually SMALLER than the 98-00 cam.
*2.73, but that’s for automatics only. For M6 cars the base axle is 3.23 with 3.42 as the performance option.