Ahoy captn-t,
I have a 521, C6 and use a 2.88 Jag diff. It's perfect for my street use and 17" tires.
Your answer of course depends on the state of tune of the engine, intended use for the car, tires used and weight.
For mostly street I'd recommend the 2.75 posi.
If you use 15" tires you'll probably be able to blow them away any time with the 460 in almost any gear. With 17" tires you still can do that, but you'll have to want to!
Regards,
Tom