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.
Dana makes the 61,71 and maybe even the 81 series with the 3.54 and higher sped ratios. The 61 first came in the Ford 3/4 ton trucks with a 3.30 ratio, later in the Dodge pickup CTD trucks as already mention with the 3.30 and 3.07
One of the local scrap yards has both, maybe I should scoop both of them up for future builds
