A couple thoughts on this situation...
First, the only difference between a "disk" and a "drum" master cylinder generally is the reservoir size and the addition of a check valve. Disk brake master cylinders usually have large reservoirs on both ends, and do not have check valves in the outlet ports. To verify if a check valve is installed or not, remove the line from the master cylinder and gently insert a small rod into the hole in the brass fitting seat in the master cylinder. If the rod goes in over 1/4" or so, there is no check valve installed. If you hit an obstruction, that is the check valve. That would be the port for a drum brake axle with no other residual valve required. If neither of the ports have a check valve, that master cylinder can be used with drum brakes, you just need to add a 10# residual valve in-line.
Second, without knowing what type of car the "GM" combination valve was designed to be installed in, it is a crap-shoot if it will work perfectly on your car. Telling the difference between a disk/disk or disk/drum combo valve (if my memory is correct) is the installation of a check valve for the drum brake axle port (it wouldn't be in the master cylinder in this case).
Third, since your car is unique in its center of gravity location, tire size combination, etc, the possibility of installing the perfect "factory" style combo valve is slim. I would chuck the combo valve, make sure there is a 10# check valve somewhere in the drum brake circuit, and install an adjustable prop valve in the rear line. (I haven't used a stand alone metering valve in my many projects, but I would look into it for future use)(I have set up many excellent braking systems without one). Set it to full pressure and test the car in an open gravel parking lot at slow speed. After warming up the brakes a bit, jump on the pedal and see if you get any lock-up. Like was stated earlier, adjust your brakes so the fronts lock up JUST before the rears in a high speed full brake application (work up to this while adjusting your prop valve).
Lastly, make certain everything is installed, adjusted and bled properly. Especially the push rod between the master cylinder and the booster. Also check that the brake pedal linkage doesn't go over-center in its normal travel. I have seen some cars that this was the case, and when the pedal moved past the high point of the arc, the harder you press the pedal the brake pressure was actually going down!