Disc brakes need a lot more pressure to work properly than the drums. Drums have a lot more contact area between the friction material and the drum, and the drum rotation against the shoes also creates a moment in the shoe that helps press it against the drum mechanically, so line pressure requirements are lower. The smaller contact area with discs means you need more pressure at the contact point to achieve the same amount of friction. The stock prop valve also has a residual pressure function to counteract the springs inside the drum and keep the shoes up against the drum. That residual pressure is too much for a disc set up and you'll end up dragging the rear brakes.
Basically, if you switch to rear discs without replacing the stock prop valve, you'll get excessive pad wear and heat buildup because it'll drag the rear brakes constantly, but then it won't stop as well because the prop valve will limit pressure to the rears when you actually use the brakes.
For four wheel disc, you need an adjustable prop valve and a 2psi residual pressure valve on the rear circuit. The 2psi is enough to prevent most pad knockback, but it won't drag the brakes like the stock valve, which is likely closer to 10psi residual pressure.
A non adjustable disc brake prop valve from a car that had four wheel discs would work, but you'd have no way of knowing if the brake bias was optimal. The only way to get it right for your car, brake system, and tire combination is to use an adjustable prop valve.
