I believe that the combination that you are using is not a combination that Chrysler ever produced. Therefore I presume that there is no "factory part" to correctly proportion between K-h discs and 9" drums. If you have problems at initial testing that shows that the factory parts don't do the job, this is a way to adjust the rear brake pressure so that the phenomenon that you fear will not happen in an emergency.
As stated in post #5 there were lots of cars with 7 1/4 rears with 10 inch brakes - almost all the 68-71 cars with 318 2bbl and an automatic. I believe all the V-8 cars had 10 inch rear drums regardless of rear axle. Since a lot of 7 1/4 cars with V-8s get switched to 8 3/4 or 8 1/4 and the rear brake don't swap (different bolt pattern for backing plate), I would bet a want ad would get you a bunch of offers for 7 1/4 10 inch rear brakes at low prices. Then it would be a factory available combination and you would probably have no issues with the stock proportioning valve(s).