Hi, are you also on AllFordMustangs? I was googling your part number and came up with something you posted about your booster brake install. You had your engine out not long ago? If so, that would have been a great time to do the brake stuff. I'm a member of stangfix.com, but haven't been on it in a long time. I'd feel like a traitor if I popped back on there!
The pic below just shows clearance is close, but that's how it was for the factory setup. It's awesome you have the factory disc/drum, so the proportioning valve that you have should be Ok? I was always told to replace all the brake lines due to the age of the car~ which I did. Something to consider.
Going now to work on my Mustang