Jim,
Bear with me on this. I have a 68 with disc/drum, I didn't post without first examining my distribution valve. Let me preface this by saying that I am a retired heavy equipment mechanic and know enough about hydraulics to be really dangerous. Gauging the orifices I found that my inlet is 1 drill size larger than the outlets. If you buy a mc booster and valve assembly the inlet to the distribution valve will be next to the switch and it is for 67 to 69. This is the inlet for a reason. The only thing common to the both front brakes is the distribution valve. The only thing that will affect the front brake operation is the fluid entering that valve, nothing else upstream matters to the front brake operation. All your components are new. Now I ask you to double check the AIM and the plumbing, at this point it can't hurt to reconfigure, you've tried everything else. Again the inlet port, even if the 3 orifices are the same, try it, you have nothing to loose but more brake fluid. You must have used 5 gallons by now. If I'm not sure about something I'll back off but if I think I'm right I can be like a junk yard dog. I really believe that if you buy a new mc booster valve set up and reconfigure the tubing to what you have now that nothing will change. I'll bet you a dinner on that. You can try this too if you like. Take one of your other distribution valves and drill the orifice to the LF one size larger. That will cause a restriction to the RF. I'm going to bow out now that I've put in my 2 cents worth, maybe a nickel. Jim maybe take a break and enjoy the holidays and get back to it next year.
You asked for help, Be careful what you wish for. All the best, Merry Christmas, Jim