Unless you are going to source a 350 trans I would go with the 700r4 since it would be only a little more work than swapping in a 400 plus the low first gear will help you get moving if you have your stock rear gear and you also have the benefit of not revving the engine really high if you go on the freeway.
And most aftermarket 168 tooth flexplates are dual bolt pattern and can be purchased for $30 from Summit Racing for non SFI versions
