The center of the brake booster is the center of the system. From there you have to adjust the length to the master cyl. It is either a screw adjustment or there are optional rods. I think your push rod is too short and is not activating the master cyl. See this for visual:
https://www.cjponyparts.com/classic-...BB9/?year=1966
You likely need one of these to get it right.
https://www.bing.com/shop/productpag...EE40C10986BA0F
PS> The rod inside the truck sets pedal position but won't help anything if the pin in the front of the booster is not adjusted properly.