The LSA is the SC'd 6.2 in the CTS-V that makes 556 HP.
The LS7, last I read, is on it's way out of production.
It seems to me the LSA is the best bet. It's likely cheaper to build because it doesn't share as many of the exotic metals the LS7 and LS9 do. Also, it's not build at Wixom, but in Mexico, if memory serves. I think the dry-sump oiling system is another drawback to the 7 and 9.
Unless a Gen V makes it before Z28 returns, I think it'll be the LSA.
Search the thread Tran suggests though. There's no concrete proof of anything, though, either way.
"...What IS true: We anticipated that this would happen - we are never finished - and yes, Ford DOES deserve to win now and then. To think that GM can come out with a car to make ford throw in the towel is simply foolhardy..." - fbodfather