[RichardJ]

The bore of the Dual master cylinders is the same in the rear (primary section) as it is in the front (secondary section).
Pressure from the rear port is the same as the pressure from the front port. Always, unless the MC is bad.

If the MC, the booster and mechanical linkage is correct for the front calipers now, then it doesn't matter if the rear brakes are drum or disk.
The purpose of the proportion valve is to proportionally reduce the pressure to the rear drum cylinders. The pressure needed for the front calipers is too great for the rear drums and must be proportionally reduced to prevent rear brakes from locking up.

Adding discs to the rear, does not require as much pressure reduction to the rear as with the drums, but because of the light weight in the rear of a truck, it may need some reduction.
Many trucks use load sensing proportion valves.

If the MC is good for what you have now, then don't screw that up. Determine after the rear disc install, what pressure reduction to the rear, may or may not be needed.