Why bother? My friend goes low 8s with a D60.
I see a few misconceptions in your post.
First, availability of gears. By "soft" I am assuming you mean 9310 gears. The Dana 60 is a truck rear end which was adapted by ChryCo for its ultra-high performance cars. In trucks you can get ratios from 3.54 through at least 4.88 in "regular" steel.
Second, it's not like the 9310 gears are just going to roll over and die. Three factors you can control are type of lubricant, mileage and driving habits. I personally don't believe you are going to drive this car enough to wear out a set of 9310 gears.
Third, on to the selection of limited slips. If you can't find a limited slip differential for a Dana 60 you haven't been looking hard enough. Chrysler used both the four pinion PowrLock and the two pinion TracLock. I'm sure there is a Detroit Locker for one. There are others, too.

The 9.5" ring gear of the 60 isn't the same as the 10.5" ring gear of the D70. As heavy as the D60 is, the D70 outweighs it and it's physically larger as well.

Email www.Doctordiff.com and get his input.

R.