I'm not sure I agree with the Ranger being "cost efficient." I went through this last year picking a tranny for my rig. NV4500 units are commonly available on the used market, while you are just about stuck buying a Ranger unit new. Price-wise, the cheapest Ranger I found was $1,376. I paid $200 for my (used) NV4500, and newly rebuilt units are commonly available for ~$1000. So I'm not sure when the Ranger makes sense, cost-wise. I know it didn't seem like a good option for my rig.
Neither would I call running 2 transmission boxes simple compared to running a single case.
The bolt pattern on the bell housing end of the NV4500 is different from what the SM465 runs, but new holes can be drilled into the face of the NV4500 to reuse your existing SM465 bell housing. Though I think this only works if you get the pre-1995 NV4500. Or you can upgrade to hydro (it's not *that* big of a deal).
for the T-case, I assume you're running a figure-8 NP205? That is one thing that would not easily work with the NV4500. But finding a T-case sounds more fun (to me) than redoing the drive shafts and fitting another box into your drive train.
Running a 6-speed sounds like a great idea. I'm betting it's just cost that keeps folks away from doing that more often.