I have made my own rear disc setup on a toyota using chevy calipers and rotors and made my own brackets. I had to get the axle mounting surface machined down so it would fit in the rotor. I didn't change out the master cylinder and the rears would lock up before the fronts.
That's the same setup I used. You can also have the center hole of the GM disks machined out, rather than machining the axle. (either way works, just depends on which way you want to go)
I just used a 1" bore master, found on later ('90-ish+) trucks, V6 trucks, and some Land Cruisers. The stock 13/16" bore MC worked, but had a really low pedal.
I had the same problem with the rears locking up first, so I added an adjustable proportioning valve in the rear line so I could dial down the amount of braking in the rear so they WOULDN'T lock up so quickly.