kelsey hayes part and proportioning valve
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 3/16x3/8 Brake line connector along with a 7/16 to 3/8 nut adapter in place of the hold off valve. I did this instead of ripping everything out and reordering lines due to my ignorance at the time.
See pics
You can see here where the extra valve would have been on a 68 or 69 a-body
Hold off valve
Simple connector installed
Easy peasy