I don't know what the old Mack's have/had. Plus its Mack, they were always different. The modernish trucks you set the gap from throwout to clutch brake at 1/2 to 9/16". Then you can set the brake squeeze and adjust for the freeplay if needed. Shouldn't have to unless someone else messed with it.  There's a couple common ones now, the easy adjust where you hold your ratchet head in with a pry bar and turn the adjuster, the value/"spicer" style where you remove the retainer and turn the pegs. Same concept, different excecutions.  IIRC there may be some with a bolt head style adjuster that has alock plate instead of being spring loaded. There's also the Solo which you probably don't have. You can remove the auto adjuster and install a tool to adjust those.  I'm not very familiar with older stuff and what all may be out there though, not that the clutch in your truck would be old, I just don't have a clue what Mack does.