454 LSX with a 6spd? Definitely don't get the 12 bolt..hah
I think either S60 or 9" will be a good choice for you. According to what I've read, the S60 weighs more than the 9", but doesn't lose as much power. But I don't think I lost much going to a Moser 9". My 1/4 times dropped only 0.1 and that was switching from 3.73 to 3.50.
On my car with 315s, I had to hammer the inner wheel well, and get a fender roller. But my car is lowered on Eibach Sportlines..