I am preparing to swap my drum brakes to a set of Kelsey hayes 4 piston discs---what is the right prop valve to use ???

My 69 GTS has KH brakes. There is a prop valve, and a secondary valve located about a foot down the frame rail from the main prop valve.

If you are not going for original, you may want to look into an adjustable one.

-Dave

Buy a adjustable good one so you can control the brakes the way you want them to be
I bought a Wildwood valve for my 1966 Dodge Coronet Deluxe and mounted on the inner left front fender well underneath the master cylinder where I can easily reach it to adjust it

There is no proportioning valve in the 67-72 KH brake system on the A body , the main valve is really a distribution block , it is the same part drum or disc , there is the added hold off valve in the rear brake line. I did a swap on a friends 67 barracuda and used the adjustable valve from Wilwood in the rear line to adjust it to keep the rears from premature lock, wouldn't be a bad part to install in a a drum car either ...