Considering the minimal input I got concerning how to separate the front and reroute the rear brakelines, I figured that it must be more straightforward than I thought. And it was.

I simply unhooked the line to the rear brakes and plugged the hole in the brass 4-way with a very short 3/8" fine-thread stainless steel bolt and a brass crush ring. I picked up two 3/16" female-female brake line connectors for a couple of bucks at Autozone and used one to connect the rear brakeline to a short patch line and into the "H" pressure distributor by the Booster. I hooked it up almost identically to Old Kombi's pic. Although I have more coils in my lines.
I then used a small clear rubber hose and a glass bottle of brake fluid to bleed the brakes. (This was a horrible and messy experience). In the end, I have very weak brakes. Stomping on them stops the car, but not quite on a dime. I may have bled them incorrectly (best I could do by myself) as they were resonably stiff at a couple of points in the bleeding process. However, I fear that the replacement rubber seals for the MC were just not up to par. The original seals were a tad thicker and taller than the replacement seals, although they seem to fit the same and were listed at Swedish Treasures as the replacement seals for the 68-70 122 Master Cylinder.

After all this work I'm a bit bummed. I'll be sure to re-bleed it before I give up, but seems like this conversion is not in the cards. As nothing was irreversably modified, I should be able to undo the changes. If anyone is fabricating one of those 12/6 to 3/9 bolt conversion brackets, please let me know. I've got a 1971 P1800 MC that I can hook up if I can get an adapter. However, another $84.00 to SwedishTreasures is beyond my budget at the moment.

On the upside, my short front driveshaft arrived today and I can hopefully get OD in the Amazon soon.