Proportioning valve options with disk brake conversion.

In the middle of front suspension rebuild on 68 Barracuda. Originally a /6 car with 9 inch drums. Upon completion will have manual front factory disk brakes and 15/16 bore master cylinder as spec. for 75 Dart with power brakes and 10 X 2.5 rear drums. With front suspension removed, noticed I had good access to distribution valve. Question, is there a direct replacement distribution valve with integral proportioning valve? If not is there a pre-fab conversion to place proportioning valve at master cylinder? Am trying not to cut into existing brake line to rear wheels for installation of proportioning valve. What about the smaller rear wheel cylinders. Is that an option?

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