Hey rob, good researching.
I am not sure of the possible diff ratio differences. GTR is 4.1 so if you can confirm 33 GTST that will tell you how much the speed will be out by. But.
You should keep your gts diff and bolt it into the gtr subframe. Just check the mounting points are all the same. This way you keep the right yoke, driveshafts and ABS fitting. Alternatively you can fit the GTR's diff centre into your casing which keeps you ABS but you will have to change to GTR driveshafts. But as you said you would need a few days off the road for a shop to do that.
Very good idea to get the cradle bushes down before you start. They are 20yo and rooted so it will be worse than current unless you do something.
You may find that most or all of the control arm bushes in the GTR cradle need replacing. I recently saw superpro do a "rear cradle" bush set for GTR for about $700. It is expensive but worth considering it will be a big improvement.
Check the ball joints where the hicas arms bolt onto the hub before using the GTR cradle. Most of them are rooted and there are a couple of hundred $$ ea from nissan to replace. Also much easier to replace with subframe out and a press.
One other option to consider....give it to a workshop for 2 days, get them to do your diff rebuild and subframe bushes, take their courtesy car and just pay. You knuckles will thank you.