[big gear head]

What do you want to do with the car? The right gear ratio depends on the use of the car. If you have a 8.2 10 bolt then the factory ratio would be 3.36, not 3.23. The 3.23 and 3.42 are 8.5 10 bolt ratios. Someone could have swapped a '71-'72 Cutlass or Skylark 8.5 into your car, so you need to know what rear end you have.

The TH350 can be built plenty strong for the 396. No need for a TH400. Just replace the 2nd gear drum and sprag and a few other parts and you have a very strong TH350 that will be quicker than the TH400, and you don't have to change the cross member and drive shaft.

A 12 bolt would be nice if you have the money. The '67 Chevelle 12 bolt is one of the highest priced 12 bolt rear ends right now. They were a one year only rear end and kind of hard to find. If you are not worried about it being an original rear end then the Moser rear ends are stronger than the Chevy rear ends and you can order it any way you want.