Not writing this to be a smart ass, but not all people are aware that the old cruic-o-matic trannys had a weird shift pattern. If you started out in the D position (the first detent after neutral) the tranny will start out in second gear instead of first.

For normal driving the shifter should be in the 2rd gear position past neutral. This starts the car out in low and shifts normally through all three foward gears. Many of shift indicators have a green dot in that location.

If you go with a latter tranny, the C6 would be the best bet. Not sure, but besides the tranny and torque converter, you will likely have to get the appropriate flex plate and starter.

Hope this helps.