as it is you have to bump the rev limiter to stay in 3rd for the 1/4, I don't think you will be able to spin the motor fast enough (at least not w/ out parts flying out randomly) to stay in 3rd if you drop the final drive gear lower by any noticable amount.
I really think your efforts are better spent elsewhere as far as gaining 1/4 mile times. Besides, shorter gearing will increase the torque seen at the half-shafts by quite a bit, and they are overtaxed already by either a 3L or an S/C, the shock loads would likely be too high for them to last.
btw, even if parts were readily available to do this swap, it would be very expensive for not a very large gain. Now on a Mustang, that's a different story entirely
