Proportioning Valve

If you have four same-sized tires on it, with disc brakes, you will need some proportioning, especially if you have a hi-compression engine and/or a manual trans.
But it doesn't have to be a factory unit (which incidentally, I have just taken apart and cleaned).
What you can do is use a split block at the front, and an aftermarket adjustable P-valve somewhere in the rear line.
But if you have big-N-littles, depending on the size difference, you may not need any proportioning at all ... or maybe you can proportion with wheel-cylinder size.

BTW;
on a DB equipped A-body, the frontmost reservoir feeds the rear brakes.
This is for two reasons;
1) when the rear brakes go out of adjustment, it will take more fluid to fill the wheel cylinders. and
2) if the rear hydraulic system fails, you will hardly notice it when plumbed this way, except the pedal will be lower.
Whereas plumbed the other way you may have a long stroke in the beginning where nothing happens. Very scary the first time it happens cuz you ,while panicking, may not remember to push the brake pedal waaay further than usual.
If the system is working perfectly, it doesn't matter.