You would need to remove the residual pressure check valve for the front brake circuit to make it a disc/drum master cylinder.
I think that you will also find that the rear wheel cylinders will need to be changed in order to duplicate the coupe brake system.

Did you check to see that the calipers were not siezed & that both the rotors & brake pads were serviceable & not oil contaminated?
If you get a Gregories workshop manual, all of this info will be available to you.

check in the tech section on brake upgrades, all you need to know is there.
im pretty sure you have to take a doover out of the front cylinder.

gday,ive just recently changed over drums to disks on a 1200 sedan.the problem im having is the pedal is rock hard and you really have to stamp on it to stop.i have a coupe which the pedal is a bit more spongey than this.is there a restrictor in the master cylinder?why im asking is ive seen this in valiants,where there is a olive in the outlet of cylinder that you take out to allow for more flow.does this cylinder have something like this or do i have to change it ?????? any help would be great cheers nuts