On my 240Z I just did a similar upgrade.
~Toyota 4-Runner Calipers
~240SX Rear Calipers
~'79 ZX Master
I re bent the brake master lines to allow the ZX master to work. A side note: on the 240Z the front reservoir is for the front brakes and the rear is for the rear drum brakes. On the ZX they are reversed front is rear and rear is front. When upgrading to rear calipers first take the guts out of the proportioning valve in the rear as it is setup for drum brakes. Break brake lines loose, take mounting bolt out and bring to work bench much easier. Put in vise, remove bolt on the bottom, remove the c-clip and pull the small valve out, reinstall the bolt. This allows more fluid to flow for the rear disc brakes. I upgraded to the '72 or later brake booster since the rod is longer than my '70. The early booster has a shorter rod and won’t push the plunger in the master all the way. You need hog out the pedal clip for it to fit and the pin to slide back into place. With the brake upgrades minus changing out the booster I had no pedal. I had minimal in the front and no rear brakes.
All is very well now. Hope this helps.