I have done quite a few of these systems from using all factory parts A6 compressor...etc to vintage air. If you don't care about originality vintage air is the way to go if you want ultimate cooling. The factory ducting is not quite that efficient with cooling. But mind you this is where it gets to 116 out here in AZ. It may be fine in other parts of the country where the outside Temps don't get as high.
It also helps to have a bigger condenser when switching to R134 and all the baffles up in front of the car.
Last bird I did was a 67 with an aftermarket under the dash unit with the sanden compressor and R134. It cools good enough when it's above 90 degrees here.