I should have thought to mention this, but there's a double-sided seal that is a direct replacement for the input shaft seal, it's part number is #14095609 and it's what Novak includes when you get a tcase-tranny adapter. Like your black RTV, it's just an extra bit of protection. I found it at Rock Auto for $4 (Novak wants $20 for it

)
That special gasket material is probably what I needed for putting on that housing when I just did mine this week, and I bet it's what I saw when I removed that housing to replace the seal. But I searched online and found instructions on the tera-low 4:1 gear set that just mentioned using RTV there... if it's on the internet, it must be correct, right?? Hope mine holds
The FSM doesnt mention the Anaerobic sealant, but the other book does. I think one of my pics shows some of the old Anaerobic sealant still on the case where one of the aluminum pieces sits tight to it. Very thin/clear type stuff, like dried up CA glue. Not sure if you really need it or not, but since I tore the whole thing apart I want to try to give it the best chance at not leaking LOL
When I had to change the rear output shaft on my Dana 20, I just RTV'd the output housing to the face of the case, no special sealant, and it didnt leak

Hopefully you will have the same luck
The rear transmission bearing lack of seal does have me kinda

It is almost like a built in leak, the factory knew about it and provided a drain hole in the factory trans to t-case adapter
I dont know if a double sided seal on the t-case input housing will help here or not. I will have to look and see, but I dont think that the tcase input housing rides flush against the rear tans bearing in the adapter housing

If it doesnt, then there will be a gap where oil can leak through. But I think it (it being the t-case input housing) must ride against this bearing or oil would just flow right out. I will take a look and see, thanks for the heads up
What brand seal did that Part # come from?
The entire factory trans to t case sealing interface is kinda crappy. Not just the 300's, the 20's are , too. My T150 to Dana 20 interface leaked all the time. When I swapped in a Ford T18, using an Advance Adapter kit, the new trans rear bearing was a 1 piece sealed bearing. The adapter plate (1" thick aluminum, I think it has a raised ridge on the t-case side that pluged into the open hole of the Dana 20 input situation) never leaked. If I would have been smart I should have looked for a 1 piece sealed bearing to replace the open ball bearing in the rear of the T176.
Well now that I think about it, the trans to t-case adapter sits flush against the rear trans bearing and acts as a "seal" against oil flowing into the open cavity where the trans output plugs into the t-case input

I will still look into that "double seal, though :-)