I am planning on installing rear disc brakes, I want to keep my original Prop Valve as i just put a new one on there, where is a good location to put a adjustable prop valve for the rear. I would like to be able to just unbolt the 2 lines and bolt them back to the adjustable prop valve. is this possible. on my 67.

 

The original proportioning valve was below the rear seat in early 67, and below the LH hood hinge later. If you mean a factory original disc brake valve, don't use it. It's preset to provide pressure to the rear drums, and will not properly operate disc brakes. Use an aftermarket adjustable valve. These will easily screw on in place of the OEM.