[Mike C]

How well do your brakes work at speed when you jump off the throttle and mash the brakes? That deceleration with motor should spike the vacuum and let you know if more booster push would help.

Try it in a 3rd gear or even 2nd same test to see if high vacuum situations make the brakes feel better.

I have manual stock disc setup in my '69 1/2 ton, and it stops HARD. On my '70 3/4 ton I converted to later model discs and kept the smaller diameter drum brake booster, and it worked well but you could tell it had a cam in it since it did stop better in high vacuum deceleration scenarios when getting on the brakes.