does anyone know a good six speed manual for a 460 Big block

T56, out of a Viper. Good luck finding one. $$$$
A Tremec would probably be cheaper.
Do you really need 2 overdrives??

T56 is a Tremec box...
The new T56 'Magnums' are meant to be a good thing, rated at 700ft/lbs of torque, they are tremecs aftermarket version of the 'tr6060' 6 speed in the FGs...

pretty much your only choice for a 6 speed that will have an off-the-shelf conversion available for it i'd think?