For the 4x4 guys swapping a 8l90 to an old style passenger drop transfer case.
Had a few pm's asking about this, so I'm just putting this out here. Sorry for the jumbled post, it's just a coy and paste from a couple conversations.
I modeled my t-case adapter off something advance adapters already makes, their part number is 50-6910. Theirs is too thin to work for what I was doing, but the bolt circle mates to the 8l90 and 205, and it has the notch already for the 205's shifter rod. I just had the local machine shop make me one about 1" thicker (roughly 2 5/8" thickness). There are other t-case options in the 6 hole round pattern with a passenger side drop (241 and 208, etc) if you go this route, but the VSS makes it a little tougher.
http://www.advanceadapters.com/image...ge/SY_0050.JPG
Use a round pattern, 32 spline input, long input shaft 205. You have to find something to do for a shifter as you lose the pivot point when you ditch the factory adapter. I twin sticked mine and run a cable shifter. And I had the speedo housing modified for a VSS.
This setup also leaves you with a rather short front shaft, which could be an issue if you're lifted. I run a 4" lift and with the engine as close to the firewall as I felt comfortable running, my front shaft is 26 1/2" long. U-joint angles become pretty severe at that point.
My 205 was fixed yoke output for the rear. I have a long bed, so my rear shaft was 65" long which can become an issue for drive shaft critical speed if you run any sort of gears (I run 4.56 and 35" tires). I had a shaft made from 4"od .083" wall steel tubing which gave the highest critical speed on spicer's calculator (short of exotic materials). Even then, I'm limited to about 95 mph.
If I were to do it over, I wouldn't use a 205, or I'd run a doubler on the 205 transfer case - anything to help with the driveshaft lengths.