I think you're on the right track.
Do only hook the trans/xfer girdle etc. to the mount.
It will all move as a unit and if you tie it down in more than one place it won't allow the whole drive train to flex with torque.
One point I need to make is what you have designed looks good, BUT you need to remove every bit of extra metal that you can or the thing will weigh 900lbs! It will be plenty strong, and you could even make some lightening holes to save some more weight.
I have seen a few D300's that had an added torque foot that captured the transfer case and prevented it from rotating with torque. However I have seen more where the D300 just was hung on behind the tranny and the crossmember was bolted to the trans only.
My Atlas was left hanging in the breeze so to speak at the recommendation of Advance Adapters. They said there was no need to control the xfer in torque as long as the trans' rotation was controlled.
My drive train is suspended by two large mounts on the engine and two similar mounts on the crossmember. The Overdrive (in my case) is bolted hard up to the cross member. The mounts form a Trapezoid with the top leg 20", the bottom leg 30", and a height of 48"looking down from the top.
This suspends the whole mass as one unit, at 4 points, so which ever way the motor moves the rest of the machinery has to go along. However the mounts only allow small amounts of movement. This is fine although it will probably transmitt some vibration to the rest of the vehicle, the the drivetrain is located positively.
Your's will do the same thing however you must not let the flexibility of the mounts get out of hand.
The more the mass can move, the more momentum it can gain in abrupt situations. The more momentum it gains the higher the possibility that something will give way, scattering parts accross the landscape.
This lesson was learned on the Moab Rim trail in Moab UT when I dropped into a little hole and the front bumper hit a rock and abruptly stopped the jeep. However the entire drivetrain did not stop, and continued forward cutting a perfect 13" hole thru the entire core of the radiator. This was momentum in action. Body in motion tends to stay in motion?
Since we were at the top of the trail it only took us 2 hours to recover the jeep and drag it down the trail back to the road, where it was put back on the trailer and towed 860 miles back to Ventura CA.
My entire Jeeping experience for that trip lasted about 20 minutes, and then of course there was the 2hour + recovery which everyone in the club throughly enjoyed.
The result was mounts made from spring bushings and a positive location of the drivetrain, backed up by a limit chain bolted to the trans and the frame preventing the drive train from going forward even in the event of a mount failure.
You would be wise to incorporate this feature also.
Randy