C6 transmission with overdrive module.

[2 FN LOW]

I have a mostly stock 351C..Ive decided to buy a new transmission cause the tranny i have now needs repaires, the sprag cluth is starting to go..I wanted to go with a AOD but from what a few people have told me, they are junk and will need rebuilding after a few months...I hear the C6 is a pretty strong 3speed...My rear has 4.11 gears which i would like to keep, i would love to have overdrive but if the AOD is junk i dont wanna waste the money..What is a good overdrive module which i could use with the C6? Any ideas on how much one would cost? Im not a transmission guy so i know nothing about this really.