The '68 GTO power box was 17.5:1.
Some consider the 12.7:1 boxes (2.5 turns lock to lock) to be a bit twitchy out on the highway. I have one and I think it's one of the best things I've done to my car ('64 Tempest) and have never had any issues with high-speed instability, parking manuevers are much easier now. Properly done with the high-effort torsion bar inside the box (no sloppy feel like the stock box) they drive great, mine was done by Lee Power Steering.
A good compromise is the 14:1 box. I found a good used one on a '79 -'81 Trans Am and it was a direct bolt-on (3 turns lock to lock), I put that box on a '64 LeMans. The internal stops inside the 2nd-gen F-body box won't allow the tightest possible turning radius when installed on the A-body cars, but the car still turns plenty tight and I really haven't noticed much difference from before in that respect.
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