I gotta say...my vote is to use the drive train from the C6 car and put it in the 4 speed car...when you can get your hands on a good 4 speed, swap them out.
Use the other parts from the C6 car to build out the 4 speed car.
I already have a rebuilt 72 toploader as part of a swap I did for the old motor out of the C6 car. Drivetrain wise I'm in pretty good shape. I'm in the process of blasting the 4 speed car to assess the bodywork.