[Ryridesmotox]

As far as interchangeability...

#1 the crankshaft reluctor wheel is 24 tooth on LS1/6 and early LS2. Not interchangeable... Can be fixed by having a machine shop press on your old wheel. Gen4 blocks use a 58 tooth wheel.
#2 knock sensors are in a different location, not a big deal, just a longer harness needed.
#3 cam sensor is in a different location.

Otherwise Gen3 and Gen4 blocks are pretty much interchangeable. Oh and the rectangular port heads won't go on a LS1/6 because of smaller bore.

The LS2 I have is a bare block. I have the L76 rotating assembly I took out of the block I am going to bore and stroke. Give you everything for $1k if you want. But I haven't had a chance to see the cylinders if the LS2 yet. Its at my buddy's shop. The LS2 and L76 share bore and stroke size. They are the same blocks. So if the bore needed cleaning up, you'd need slightly larger pistons. The LS2 is currently used as a fitting piece, to locate motor mounts and intakes and such in custom setups.