The ls2 officialy starts with the introduction of the 60-2 crank relucator wheels. I forget which of the GTO's had the 6.0 with the 24x wheel but they did exist.
All 2005 and 2006 GTO's were LS2's.. no idea where you are going with the "not really LS2" thing. heads, cam and block were all the same. pan, exhaust manifolds crank pulley and water pump were different than the Corvette LS2.
IIRC, 2000+ LS1 blocks are stronger, Like Sean said, the perimeter bolt heads and rocker covers are kinda unique, but you can update to the center bolt heads and valvecovers, it all interchanges.
When the GenIV came out, the dropped the LS1/LS6, so 5.7L's went away. LS2's came in in 2005 on the Corvette and GTO, 2006 in the TBSS and SSR. In 2007, the L76 came in, the truck ones had a different block casting than the car L76 (used in the G8). The Car casting is more desirable because it has holes in the main bulkheads for bay-bay breathing, and is worth over 10 (OEM tested) HP at engine speeds over 5800 RPM or so.
I can go on for pages on the differences year-year, but you need to ask specifically what you want to know.
FYI, the piston slap noise, while annoying, never caused a premature failure. GM tightened up the piston-bore clearance, added coatings on the piston skirt, and went to full-floating pistons. That pretty much took care of it. The LS3 Grand Sport I am borrowing for the weekend has no sign of it (started it at 55 degrees the morning), but the 4.8L I put in my 93 Sierra does when it's below 50 degrees outside. BTW,the 4.8L still runs pretty damn strong for the smallest motor in the family, and towed my Caprice on the trailer at 88 mph in OD.. I kinda had the speedo cal a little off when I first built the truck.
You really can't go wrong with any of the LS engines though. Just pick what your ultimate power target is (and budget), and start asking q's. For example, a 5.3L iron block engine is freaking cheap to find, and makes 295-315 HP stock with 330+ ft-#. It out powers a stock LT1 by a long shot. If you want a 500 HP ride though, I'd start with a 6.0... you can see complicated it gets if you don't narrow your goals a bit.