The big difference's between a drum Master and a Disc Master is A) The size of the reservoir....Disc brake calipers require a larger amount of working fluid than a drum wheel cylinder, thus a larger reservoir on the disc side of the M/C. A disc system will run 800-1000 PSI to pinch the rotor, a drum 400-600 typically to push out the shoe's. B) Residual Valve is another difference...All depends on the year/brand/design/combo, but some MFG's had a residual type system built into a drum/drum distribution block, some in the line port of the master, some had none. With that aftermarket Master Cylinder it's hard to say. Looks like there's a couple blue residual valves that come with the kit. Also looks like the reservoirs are identical in size.
As far as putting a disc/disc on a disc/drum I guess it's a possibility. A drum reservoir on a disc is definitely not. One would think you could just plumb to a standard distribution block (not a Proportioning valve). From there leave the the lines for the fronts alone. Install that adjustable proportioning valve from the kit in the line leading back to the rear brakes. Being you'll still have drums in the back, you will also have to install a #10 residual valve. What that will do, is when you release the pedal the return springs in the drum will pull the shoe's back into a resting position. With that being said, the fluid will be discharged back to the master. You don't want all the fluid rushing back up into the M/C, so a residual valve is installed in the line to the rear brakes somewhere. Be sure to use a #10, not a 2#. The #2's are used on a disc set up when the master is lower than the calipers (like in a street/rat rod configuration). When you do convert to disc in the back, you will need to remove that residual valve. If you don't, the caliper will not fully retract and drag because fluid is being held by the residual valve and there is no return springs to pull the pads back to the resting position with a claiper. You'll also have to reset the position of your adjustable proportioning valve to the rear. Again, Disc Calipers require a lot more fluid for movement (pedal swing) so once the disc's are on the back of your ride, you may need to open the valve a bit more to allow more fluid to the caliper and achieve the typical 70/30-65/35 braking exponential front/rear
As far as the booster goes, that one shown might be designed to handle both types of applications. Our old OEM stuff is a different ball game. Either way I would still ask if the combo's share the same booster before you go buying.
Good Luck
