It turns out that Advance Adapters has the coupler as
@bucket suggests. They say they'll have them in stock in late May. And now Offroad Design also says they'll have them in late May! For everyone's reference, here is the coupler from Advance:
1971 to1984/85 GM SM465 to NP205 (figure 8) 10 spline x 10 spline stock replacement power sleeve / coupler.
www.advanceadapters.com
Earlier in my thread, I had a lot of questions about how the SM465 and NP205 were bolted together, and how they were bolted to the frame. There is an adapter that goes between them after all. I've now got the SM465 and the adapter on my bench for cleanup and new gasket application. SM465 seems to rotate well with no binding, and it worked well when I was driving it. So I don't think I'll poke the sleeping dog. The only reason I took it all out was because of various broken bolts, likely caused by the missing adapter mounts I was talking about earlier, and massive gear oil leaks, as evidenced in my pictures.
In any event, I thought I would share this excellent page from Advance Adapters, which would have answered a lot of my questions up front.
1. GM New Process 205
www.advanceadapters.com
I have the gasket kit for SM465 now from Offroad Design. I also have their kit for the gaskets between SM465 and adapter, and between adapter and NP205. Does anyone recommend using RTV or other product, or just apply gaskets to clean surfaces with no other product?