I use the sierra for parts alot and as said above " a few tweeks and its a new model.
The rear should go on no problem and the sub frame used to mount the diff and swing arms should be the same.
Watch out if you remove the rear subframe to make life a bit easier, the large bolts near the front of the back wheels used to mount it will be very rusty and so, very tight.
I use a breaker bar with a scafolding tube as an extension to get them out but it will take ages.
Dont be to heavy with them as the theaded part of the chassis (a nut welded to the chassis, but inside and unreachable) has a tendency to snap off.
And what ever you do, dont use an inpact gun, they will snap!
The brakes should be an easy upgrade as the mounts from the donor car should be used and should go straight on.
The rear calipers are cable opperated for the hand brake and oil for the foot brake.
For the foot brake side of things, T off the front brakes and run lines to the rear, put a proportion valve in the line so that you can adjust the presure evenly.
There may be lines there already i have never looked, this is what i do when installing it in a kit car.
Ive got a diagram for the different ways you can install brake lines. Give me a call if you want a copy.