Front disc conversion

You don't now "have" a proportioning valve. You have a distribution block which also contains the piston/ shuttle warning switch--that fires with differential pressure between front and rear

You COULD just add an aftermarket valve going to the rear brakes, and as far as the master, you may not have to change it. The rear outlet, which feeds the front brakes--likely contains a residual pressure valve, and you can rupture or remove that for disc brakes. I'm not sure about master size

If you want to do this "as factory" you need the factory prop valve and master