I am changing a 74 from vacuum brakes to manual. What needs to be changed over to use a manual master cylinder from another 74?

 

The booster goes and get the prop valve from the manual too. You got an 87 t/a or Buick? I'm in to both? Welcome.

 

I'm pretty sure the proportioning valve doesn't need to be changed. Just remove the booster and bolt the master back to the firewall. Might have to rework the lines a little to get it moved back. Might need a different operating rod.
I went the other way and simply installed a booster and operating rod.

 

I have a Turbo Buick.

 

I think you have to move the pushrod to the other position on the brake pedal too. It gives better leverage. Love the GN's.

 

Yes the pushrod needs to be moved or the pedal height wont be correct, or give proper leverage.