I ran into the same problem. I ordered pre fab lines for a 68 dart not knowing it had a separate hold off valve for the rear drums.
If you order a 70 and up you can just plumb a regular integrated proportioning valve as pictured.
Since I wasn’t going 100% 1968 factory correct, I just added a straight connector in place to the hold off valve. I did this instead of ripping everything out and reordering lines due to my ignorance at the time.
See pics
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Hold off valve
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Simple connector installed
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Easy peasy
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