DiffTechnics is a sponsor at
LS1GTO.com. You should get some good answers if you post your question in the Drivetrain section over there. I don't know the cost, but quite a few people have upgraded to 391's. I don't know if there are 411's available yet, but from what I've read you may need to swap out the driveshaft to go that high. In either case I think you would also need to get a tune to recalibrate the speedo.