Drum to disc

Drum brake MC's have a residual pressure valve in the outlets of the ports. I believe you can remove them (in your case, the port for the front brakes). IIRC it will cause front discs to drag. Also, disc brake MC's have added fluid capacity for the front brakes. If I were you, I would order a new or rebuilt MC from a parts store for a 73-76 Dart with power discs. (probably around $35) It's pretty important to have good brakes and a new MC will be one less thing to worry about. You can use your manual brake pushrod and run the new MC without a booster. An easy way to remove the pushrod from the old MC is to look at the brake pedal arm and see what is in the way of pulling it back (like the brake switch) towards the steering wheel. Use your hand to pull the pedal back and the pushrod will pop out of the grommet that is inside the piston bore. Then remove the old MC and the special bolt that connects the rod to the brake pedal. Otherwise, it can be really hard to separate the rod from the piston.

The bore is 15/16" diameter and you will love the feel. No reason to have power brakes on an a-body unless you have some physical limitation in your braking leg. Power brakes do not shorten stopping distances.