I am going to take a gander at answering your question.
1. Are you still using the proportioning valve from the 72 nova?
2. Your new master cylinder is setup for disc brakes front and rear.
3. You must have a 10lb residual valve for the rear brakes in your setup.
4. You should also have a 2lb residual valve for the front brakes.
The proportioning valve adjust the pressure between the front and rear brakes so that your rear brakes do not grab before the front, this maybe bad and causing your problem. If you do not have a 10lb residual valve for the back this will cause your rear brakes to grab continuously and/or feel like the e-brake is dragging. The 2lb residual for the front brakes keeps a constant pressure on the front. If anyone of these problems are not working correctly you will have problems. Now there are some people here that may disagree with my thought process and I will not question them but I would check your system out. A place that has good prices that I am fond of is on this link---http://www.zigsstreetrods.com/zigs.html
Luck to you