The dana being referred to is actually a 71. No real good parts available for it.

I would (and have) gone with the strange ultra case. The standard ultra case takes a true 9” gear, but the pinion shaft can be upgraded to a large 35 spline configuration. This particular setup will allow down to a 3.40 ratio. If you upgrade to the 9.5” gear set, you can get a ratio as low as 3.2. These are pro gear sets.
Running low ratios is extremely hard on everything in front of the rear end. Plan on spending a whole lot on trans parts.