A quick take-up master cylinder works exactly like a fast pump floor jack. The first stage moves a lot of fluid volume but only at very low pressure, so it doesn't have a lot of force. Once there is resistance in the system (i.e. when the pads hit the rotor and/or the shoes hit the drum), the fluid stops "flowing" and begins pressurizing to give you the force, and that's where the smaller secondary bore comes into play. It's so dirt simple I wish I'd thought of it.
It gives you the flexibility to move a relatively large amount of fluid volume with minimal pedal travle and still generate the pressure you need to make the brakes work without excessive pedal force or monster boosters. Keep in mind that those calipers (and the pads for the calipers) are designed to function optimally at a certain pressure. Unless you know what the F550 MC bore is, we're shooting in the dark. Also, correct me if I'm wrong, but you're planning on running front F550 calipers on all four corners, are you not?
I have to assume that you have the stock vacuum booster on your '81 Chevy since you haven't said otherwise. Since I'm fairly certain that the hole spacing and pilot register on the F550 MC is not going to line up, here are a few ACDelco part numbers for QT MC's of that era.
85-95 G35, Vacuum Brake System
31.75mm & 40.0mm ACDelco#18M348
85-94 G25, Vacuum Brake System
28.6mm & 40mm ACDelco#18M316
I have no idea if they'll bolt up to your booster, so you're on your own there. I'd think you'll have better luck Chevy to Chevy but I've been wrong before.