I think you are on the right track, but what the heck, I might as well jump in and confuse things.......
Of course, I'm running a Ford, so there is no direct correlation, but they might have had a better way. Might be worth a look. I ran a '79 F150, C6 transmission and NP205 for 10 years under pretty darn extreme conditions with no cracking or damage to the tail of the transmission.
I have no idea of the strength comparison between the case of a C6 and a Turbo400.
I also had a gear drive PTO on the side of the 205 driving a big honking winch. I do know that there was a side mount support on the 205. Driver's side, of course, since it was a Ford.
When I bought my '89 F250 new, it came with a BW1456 aluminum transfer case. With no PTO opening.
So, I swapped it out with a 205. Then I hung a big heavy PTO adapter and very large and heavy hydraulic pump on the side. I worried about damage to the tranny, so I went to a junkyard and found the side mount off an older Ford 205.
When I started to attach it, there was no way to mount it to the frame. Since the new trucks did not use a 205, they had retasked the mounting holes in the frame for some fuel stuff.
So, I built my own. I won't go into the details unless someone is interested, but I floated the mount between two sets of rubber bushings. The transfer case was free to twist a small amount, as well as move some from side to side.
I unbolted the whole case and pump combo and weighed them. Then bolted it all back up and jacked up the transfer case/transmission with a scale until I saw that much weight on the scale again. That meant that I had removed the weight of the system from the transmission.
I then built my mount so that there was no load on the bushings.
When I released the jack, everything settled some, hopefully spreading the load equally on all the mounts.
Did I do right? No idea, but that system has been in place since 1989, with no problems or structural failures.
Does this help or hurt you? No idea. BUT, here is my suggestion. I am basing this on my memory of a mount I saw for an hour or two 30 years ago. But, as I remember, the Ford factory mount was a multi-piece mount with rubber built in. It looked like an engine mount, and there was movement between the part that bolted to the transfer case and the part that bolted to the frame.
I am wondering if it was designed to have the same amount of flex as the other engine and transmission mounts.
Where you would find one or even a picture of one, I have no idea, especially since most of the parts you folks have laying around are GM. But I have heard a lot of talk about using Ford front axles and using a driver's side drop transfer case, so someone might just have a factory Ford frame mount.
I actually like the idea of rod(s) from the top of the 205 casing to the bell housing bolts to take the weight load of the 205 off the tail shaft and transfer it to the bell housing. They need to be carefully measured as to length, and if I were doing it, I would tend to mount two. One from the left and right hand top of the transfer case to the bell housing bolts.
But then I run Fords and hang out here, so my ideas tend to be weird...........