FWIW, I always advocate removing the non-adjustable proportioning valve, for the simple reason that it was designed for Goodyear Arriva belted bias tires or occasionally, Michelin XWX radials, both in rock-hard compounds and tiny contact-patch-sizes compared to what's common on most Panteras 40 years later. And a large number of cars now have bigger rotors, bigger calipers or Porterfield pads with much better frictional qualities than stock. Tires, rotors, pads and even adapting a different master cylinder or brake booster will all vary braking force at the tire/road surface.
Bottom line: if your Pantera has significantly different sized tires from stock, or different calipers or other brake parts, the stock valve is guaranteed to be set wrong, and cannot be adjusted.
As someone said, a sensible test is to remove the valve and try panic-stopping a few times in a deserted parking lot (for safety). If you like the improved braking action with no valve at all, fine- you're done except to get used to the improved stopping power.
But if with the valve gone, you CAN lock up a wheel or two and stability is iffy, an outside observer can tell you which one it was. Then if you wish to go a little further in optimizing your brakes, you can add a manually-adjustable proportioning valve and adjust it for your car with your tires and your brakes. If you later upgrade any of the parts involved, your manual valve can be readjusted to compensate.
One caveat: don't adjust your new valve for a ragged-edge brake balance. Road surfaces vary in friction and if you find yourself on concrete, or damp pavement or dusty blacktop, the friction may be different from your test surface, and again you'll be locking up one or more wheels- often at an inopportune time. Give yourself a little margin for changing conditions.