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 ...