'68 Galaxie trans swap questions

Hi, I have a 68 Galaxie 500 with a 390 in it that is about 80% original and am looking for some options on a new transmission. The car has a perfectly fine C-6 in it I recently rebuilt back to stock. The question I have is I'm looking for some options on overdrive transmission options. The reason I ask is the car has a little over 400 hp and somewhere around 480 torque and am relatively new to the FE world and am unaware of any OD transmissions. Any ideas?

Thanks