I did this to my 65 Dart, plus added a vaccum booster so I can easily add front disks later. I put a booster/MC from a ~99 Breeze on Dart stand-off brackets. The 4-hole manual MC from a 68 Dart should bolt up. Many use a later 2-hole alum MC on a $50 adapter plate (see posts).
For drum-drum brake cars, the distribution block had just a pressure imbalance sensor between the front and rear lines. It lit a warning light, which you don't have. In many cars it became stuck with rust and didn't work anyway, and is tricky to adjust. In sum, you don't need it.
Front disk cars, had an additional proportioning valve (pressure reducing) in-line w/ the rear brakes. It was a separate piece in early dual MC's and by 1969 (earlier?) integrated into the distribution block. You only need that for front disks. Best to get a new $25 adjustable one like I did and mount up high near the MC.
As mentioned, use your 65 block for just the front brakes and plug the rear port. You can get a 3/16" inverted flare plug and a 1/4" male to 3/16" female adapter in bubble packs at Autozone. You need the later if your MC has a 3/16" tube as all (?)later ones did. Just because the nut fits doesn't mean the fitting is correct. Also get a 3/16-3/16 union for the rear if connecting to the existing tubing (I did), or run new tubing.
Let WD-40 soak in for a few days and work the nuts back and forth. Otherwise, count on shearing a tube and have a dbl flare tool ready to repair it.
