I probably won't be much help here, but I installed a good running 69 302 cu.in engine, stock with a 2 bbl. carb, and its factory C-4 trans. I am retaining the stock 9" rear, but may change gears for a higher ratio. It is not running now, but it did run well in all aspects.
I was looking for reliability, and simplicity. I don't care much for the newer injected, computerized engines and trannys that many are installing these days. I have grown too old to adapt to the modern day technologies.
Best of luck on your choices for replacement driveline.