c6 would be the toughest choice on automatics for a 351c, and for an overdrive you would end up spending about $1500 for the overdrive module which actually connects between the tranny and the driveshaft, almost like a transfer case.... some models can be setup to give you a 6 speed with an overdrive at the end and puts gears in between your first and second and second and third....i believe those models are a little more though