I did this conversion several years ago following Jim Cannon's (he is the 1963 Tech adviser for the ROA) conversion. Use the master cylinder for the 1967 Riviera, it is a bolt on. You can find either of the versions at your local auto parts stores. I got mine from NAPA. I kept the original booster as you are doing. His conversion is pretty straightforward and simple. I have not had any problems whatsoever.
Like anything, there are multiple ways to do it.
Here is what I recommend:
Keep the factory distribution block on the frame. Remove the line going to the rear axle and plug it. Connect the port on the block that goes to the stock MC up to the FRONT port of the dual MC. You need to fabricate a line to do this, with double-flared ends at both ends. Put some slack in the line by making a Z-shaped portion or a coil The tube nut size on the front MC port is different from stock MC, but is easily obtained. That takes care of the front wheels.
For the rear axle, you want to fabricate a new line that goes from the REAR port of the dual MC down to the frame in the vicinity of the distribution block. Include a coil or a Z for slack. Use a T-fitting style union made for brake light switch to connect the end of your new rear line to the existing rear line that come off of the original distribution block. The 2 union-type ends will be tube fittings. You connect the line to the rear axle to one of them and the line up to the dual MC to the other. It is equivalent to a straight-through union. The T-part of the fitting is 1/8" NPT, not flared tubing. The stock brake light switch screws right into it. The wires on the stock wiring harness are long enough to reach down to the frame by the distribution block. You do not need to splice any electric or cut anything.
To keep the lines from vibrating against each other, I tied the lines to each other with a couple of small zip ties.
I got the 3/16" tubing, the master fittings STN-7 and STN-5, the T-fitting BS-01(the T-fitting comes with a new brake light switch attached) and everything from Inline Tube. I might have gotten the plug from them, I don't remember. Call them.
You do not need a proportioning valve for the drum-drum system. Crude proportioning is already included in the wheel cylinder cup/piston diameter difference between front and rear. Connecting lines as I describe will maintain the factory proportioning, for better or for worse. (I used to design brake systems.)
The brake light switch is hard to bleed air out of. Fill it with fluid before screwing into the T-fitting. Try to keep it contacts down until you bleed the system, then invert it.
Regarding which dual MC to use:
You need to remove your stock MC from the car and look at the end of the piston that the booster pin presses against. It will be either a (roughly) 1" deep hole or a 1/4" deep dimple. The new dual MC that you install needs to have that same depth "hole". It will be either one or the other. Trying to put the wrong one in will not work.
If you are keeping your STOCK booster, you need to put an o-ring on the neck of the new MC where it inserts into the booster, or else you will have no boost (due to vacuum leak at the neck). Either move the original o-ring over to the booster or get a new o-ring. The only guy I know that sells this special o-ring is Booster Dewey out in Oregon. You might try NAPA or other places and find it. If you were to "cut" the o-ring and look at the cut surface, it would not be a circle. It would be a small square. So it is not really a conventional o-ring. It is specific to this application.
The write up on the ROA website is essentially the same as what Jim outlined but, somewhat more complicated. With the above method you don't have to replace the distribution block and fabricate a mounting bracket, your original distribution block is utilized. Also, if you are doing this on a 63 you do not have to mount and rewire a new brake light switch as outlined on the website. An original style brake light switch is included with the T from Inline Tube. The wires that ran to the switch on the original master cylinder are long enough to reach the new switch in the T.
Hope this helps.