Flash has it covered pretty damn well. Since you don't have any manual parts, you aren't painted into a box like some of us.
Your car is slammed lower than pretty much anybody. You could definitely use the tunnel clearance to decrease driveline angles. I'd go 56 mag and cut if you have to. It does have the highest torque capacity and modern technology. I also like the slightly taller first gear along with the 2nd overdrive. Do you really need two overdrives? Not really, but it would allow you to drop the rpm back down into the 2000-2500 rpm from 90-110. You can run the LS motors at pretty low rpm in high gear as well. I get away with down to 1700-1800. I bet you can do 1500 easy.
If you don't want to slice and dice to get reasonable working angles, a modded 600 or rs600 is a better bet. You can use a mechanical clutch with any of the three now.(Different Bellhousings) You'll also have more chances of picking up some deals or the latter...
I hate to recommend a modded 600 without experiencing one first hand. My finger is still itchy to pull the trigger, I'm just not that excited about pulling the trans. The car is working so good right not. I'll likely go to an aluminum flywheel at the same time.