2nd gen Cuda brake help. Front drum to disc.

Well, one problem may be the wrong master cylinder. The factory disc/drum master cylinder has a larger reservoir for the front brakes. Is that the same master you used for drum brakes? If so, it would have a residual pressure valve in the front line, which would cause the disc brakes to drag. Your proportioning valve is also in the wrong place. It should be between the splitter/safety valve and the line running to the rear of the car, not between the MC and the splitter.

I have performed this conversion successfully on my convertible. I did that by matching the factory disc installation on my fastback.