I need more boost from my Brake booster Which year did GM go do dual diaphrams?

[Chevy Muscle]

I put disks on the front of my '69 K5 and I replaced my M/C, prop valve for the correct disk/drum versions.

However, my booster isn't boosting enough and I have a very hard pedal. I'm getting 16" of vacuum at slow idle and over 20" of vacuum at driving RPMs.... but still too hard.

I want to replace my booster. I've been offered used ones from a '74 3/4 ton and one from a '87 1/2 ton truck.

Main question: What year did GM improve the design of the boosters? (go to dual diaphrams?)