Proportioning valve

Do you have four same sized tires on the car?
If yes then any valve off a disc/drum A-body will work.
You can fine tune it, if you need to, for your application with one of three sizes of rear wheel cylinders, or by the use of braking materials.

I started with a 73 disc/drum C-valve, and as my rear tires got progressively taller and wider, I eventually just gutted the proportioning part of the block.