The proportioning valve will regulate how much pressure goes to the front vs. the rear. Since discs require more pressure, using a proportioning valve set up for drums may leave the rear discs not getting enough pressure. This would leave the front to do more then their share. I'd probably run it like it is then if you need to change things you could remove the pill and spring that regulates the pressure. Then if you really wanted to regulate it more you could put a inline adjustable proportioning valve in. Unless you're running TJ/ZJ discs you'll probably want a bit more pressure.
As for ABS, how much of the Jeep was the ABS controlling. (ie, was it 2 wheels or 4 wheels?) You can usually pull the ABS fuse to disable it from what I've been told. If you really wanted to get rid of it, you could swap parts out and convert it over to a non-abs setup. You could probably plug the additional ports with screw-in caps.