Original proportion valve from a 75 Duster do I need that plastic tool to bleed the brakes? Or is that for the newer-style valves?

If it has the brake light switch built in you need to hold the piston centered while bleeding. If the piston shifts you will never get fluid to whatever side is blocked. You dont need a special tool, use a drill bit or any pokey thing that will fit in the hole. You should not get fluid when the switch is removed. If you do, the prop valve needs new o-rings.