20:1 box with standard pitman and idler arms and appropriate tires: fantastic setup that makes you wonder why the hell they didn't all come that way from the factory.
16:1 box: efficiently drains all the fun and pleasure out of driving the car.
I agree with Dan. In defense of the corporation, they made one ratio in the beginning and it had to work across the board, A's, B's AND C bodies.
I have a 68 small block cuda with a factory 16:1 manual box. 215-60x15 tires. Great on the road but tough parking. I would not want to autocross it but, WTH, I'd give it a try.
My 64 /6 Val vert has 24:1 manual. Easy at all speeds but too slow. You just have to crank the wheel too much on turns.
Also have an original Direct Connection 20:1 worm and sector in my stash and a couple extra boxes. That may end up in the 68 with the 16:1 going in the Val as it's definitely lighter.
I'd say the 20:1 is the best all around and would be my first choice, in my opinion.