First the disclaimer: I don't mess with automatics! That's just how I am.
But it seems to me in a conversation I had no interest in, so was just half listening, I was told that there is a version of the 700R4 that bolts directly to an Atlas (so also D300) and all of the 4.3L GM motors. The topic seemed to be the best way to stuff a 4.3L in your Rock Crawler and keep the Atlas. It also seems to me that if it fits a 4.3L it will also fit the newer SBC V8's.
While is would be logical at this point to give you a link to Advanced Adapters, well ok I will.
Advance Adapters Atlas Transfer Case
They make the Atlas... pretty much a D300 improved and gone to the next level. Some people say it's the only way to go... while others say it's grumpy and hard to shift.
Now my point is that there are lots of D300 cases out there just laying around but are compatible or adaptable to late model automatics. While more complicated the D300 takes the same space and provides a bit smoother operation over the D20 (best I can remember - so I'm told).
I haven't driven anything with a D20 in years but have a many D300's and one grumpy Atlas.
Now here are a couple of more links to help you figure out where you want/need to go.
Adapting the TH700R4 / 4L60 to the Models 18 & 20 Transfer Cases
Adapting the TH700R4 / 4L60 to the Model 300 Transfer Case
As stated but could be wrong... there is a TH700R4 version that will bolt directly to a D300 (or Atlas), while some of the TH700R4 versions need a conversion kit. Why else would Novak make the kit.
Anyway... I'm guessing that if you poke around the web sites and maybe even call Advanced Adapters and/or Novak Conversions you will have so much information that your head will either explode or glow with knowledge and direction.
Do not waste any time thinking about it if you want to rebuild a D300 and need a parts kit. Hands down best price for bearings, gaskets and the most complete kit I have seen anywhere, use the Novak site and just order it. Wish that I had my search hours back on that subject.
Have I rambled enough?? Well I hope enough that you find the setup that you want.