As I mentioned in other threads, I do not believe that transmission is as much of a problem as people make it out to be. Look at it this way, a stock LS4 will make about the same power/torque as a supercharged 3.5 and they seem to be doing OK.
And if too much torque becomes a problem, there are other methods of dealing with this:
- transmission upgrades (Level 10 style)
- swap to a 5sp
Now, let me show why I am so hang up on this LS4 motor:
http://www.ebay.com/itm/08-09-IMPALA...1cbefb&vxp=mtr
$875 for a used LS4 with 88k miles... And these guys happen to be 2 hours from me...
Some people can't find 3.5 engines that DOUBLE of this price!
More info on LS4 from Fiero guys (which have the same engine bolt pattern as we do):
http://www.fieroaddiction.com/LS4/index.html
http://www.fiero.nl/forum/Forum2/HTML/113207.html
This is looking more and more promising! Though, starter is looking as one of the (common) problems.