65 Valiant small block with Heddman fenderwell headers. Need suggestions on master cylinder upgrade

When you open a Bleeder they go to-the-floor I don't care how many ports you got.
If they do, you've got something wrong. It's not up for debate, sparky. That's how it works. That's why dual reservoir master cylinders were mandated in 1967. You're right though. On a single reservoir master cylinder, no matter where you open the system, they go to the floor.