RE: Few Questions
A CJ T18 will bolt right up to the 258. What was it behind originally? The AMC V8 and I6s share the same bolt patterns.
The "Texas" style mounting flange for the Tcase is what's going to be the pain in your butt. The only drivers drop Tcase that pops into mind would be a Spicer20 from a first gen Bronco.....and I don't know if it would work.
The flipped D300 is a sweet setup. A D300 is just plain cool anyway, but being able to flip it and use it on the "wrong" side just makes it even cooler. If you go with the D300, a heavy duty output shaft is a must-have upgrade with a granny low transmission and 39" tires. They make kits to swap the D300 in place of the D20, but I don't know about the compatibilty with clocking rings and such.
The best bet would be to call Novak and ask for their opinion, chances are good that they have dealt with it.