[RichardJ]

If the front disc brakes are stock and the Master/booster are stock and the front brakes do 60% - 70% of the vehicles braking, why would you want to screw that relationship up.
Replace the old bad MC with a new replacement.

Instead of changing everything, make the new rear disc fit what you know already works.

Disc brakes require more pressure than drum brakes, so if you have any problem it would be that the existing Combination Valve/proportioning valve may be reducing the pressure to the rear brakes too much. By that I mean, reducing the pressure to the rear in relation to the pressure going to the front.

If you have to do anything it would be that you replace the Combination Valve with an independent Metering Valve for the front brakes, plus an independent adjustable Proportioning Valve for the rear brakes.